Download Basic Java Expressions and Statements

Java Programming
Basic Java Expressions and
Statements
Cheng-Chia Chen
Transparency No. 2-1
Basic Java Syntax
Contents
References : chapter 6,7,11.
1. Java Operators and Expressions
1.
2.
3.
4.
5.
Arithmetic and bit operations
comparisons
logical
assignments
others
2. Java Statements
1.
2.
3.
4.
variable declarations [and initialization]
expression statements
block statement
control of flow statements:

if, switch, for, while, break, continue, return.
5. try, throw, synchronized statement.
3. Array
Transparency No. 1-2
Basic Java Syntax
Operators and Expressions
 Arithmetic operators
 +,-,*,/,% (binary)
 +,- (unary)
 pre/post Increment/Decrement operators : ++, --
 String Concatenation Operators
 +
 Comparison operators
 ==, !=, < ,<=, >, >=
 Boolean Operators
 &&,, ||, !, &, |, ^
 Bitwise and shift operators
 ~, &, |, ^
 <<, >>, >>>
 Assignment operators
 =, +=, -=, *=, /=, %=,
 &=, |=, ^=, <<=, >>=, >>>=
Transparency No. 1-3
Basic Java Syntax
Operators and Expressions
 The conditional operator
 ?:
 The instanceof operator
 Special operators:





Object/class member access(.)
Array element access([])
Method invocation(())
Object creation(new)
Type conversion or casting( () ).
Transparency No. 1-4
Basic Java Syntax
Arithmetic operators and expressions
operator
+*/%
type
unary (prefix)
binary,
binary
++ --
unary
meaning
unary negation
binary addition, subtraction
multiplication, division,
modulus (remainder after
integer division)
(prefix, postfix) increment,
decrement (e.g., a++ is
equivalent to a = a + 1)
ex:




“total” + 3 + 4 // =“total34”
7/3, 7/3.0f, 7/0 // = 2, 2.333333f, arithmeticException
7/0.0, 0.0/0.0 // = Infinity, NaN.
7 % 3, -7%3, 4.3%2.1 //=1, -1, 0.1. x%y = sign(x) |x| % |y|.
Transparency No. 1-5
Basic Java Syntax
Example:
class ArithmeticOperators {
public static void main(String args[]) {
// Demonstration of arithmetic operators
int anInt = 10;
out.println( anInt ++ );
out.println( anInt-- );
out.println( -anInt );
// We can declare variables at any point!
int anotherInt = 3;
out.println( anInt / anotherInt );
out.println( anInt % anotherInt );
} }
Transparency No. 1-6
Basic Java Syntax
Expression
 How many (sub)expressions are there in the
expression:
 out.println( anInt
++ );
 Ans:
out.println(
anInt ++ );
Transparency No. 1-7
Basic Java Syntax
comparison (or relational) operators
 operator type
meaning
 >
binary greater than
 >=
binary greater than or equal to
 <
binary less than
 <=
binary less than or equal to
 ==
binary equality (i.e., "is identical to")
 !=
binary inequality (i.e., "is not identical to")
x == y return true iff
1. same primitive type and same value, or
2. same reference type and refer to same object or array, or
3. different primitive types but equal after conversion to the
wider type.
Note:1. +0f = -0f; NaN != NaN; NaN != any number
2. <,<=,>,>= apply to numeric types only.
 b = true > false ; // error!
Transparency No. 1-8
Basic Java Syntax
Boolean Operators
Operator
type
meaning
&&
binary
conditional AND
||
binary
conditioanl OR
!
unary
logical NOT
&
binary
logical AND
|
binary
locigal OR
^
binary
logical XOR
1. &&, || and ! can be applied to boolean values only.
=> !0, null || true, 1 | 0 // all errors
2. & and | require both operands evaluated; && and || are short-cut
versions of | and &..
 a1=0; if (a1 == 1 & a1++ == 1) { }
// a1==1
 a1=0; if (a1 == 1 && a1++ == 1) { } // a1==0
Transparency No. 1-9
Basic Java Syntax
Bitwise and shift operators
 Bitwise operators: ~, &, |, ^




byte b = ~12; // ~00001100 == 11110011, -13
10 & 7 // 00001010 & 00000111 = 00000010 or 2.
10 | 7 //00001010 | 00000111 = 00001111 or 15.
10 ^ 7 //00001010 ^ 00000111 = 00001101 or 13.
 Shift operators: <<, >>(SSHR), >>> (unsigned SHR)








10 << 1 // 00001010 << 1 = 00010100 = 20 = 10*2
7 << 3 // 00000111 << 3 = 00111000 = 7 * 8 = 56
-1 << 2 // 0xffffffff << 2 =0xfffffffC = -4 = -1 x 4.
10 >> 1 // = 10 /2
27 >> 3 // = 27/8 = 3.
-50 >> 2 // = -13 = -12 –1 = -50 /4 –1 = -50 / 4 – 1.
-16 >> 2 // = -4 = -16/4.
-50 >>> 2 // = 11001110 (204) >>> 2 = 00110011 = 51.
Transparency No. 1-10
Basic Java Syntax
Assignment operators













operator
=
+=
-=
*=
/=
%=
&=
|=
^=
<<=
>>=
>>>=
type
binary
binary
binary
binary
binary
binary
binary
binary
binary
binary
binary
binary
meaning
basic assignment
a += 2 is a shortcut for a = a + 2
a -= 2 is a shortcut for a = a - 2
a *= 2 is a shortcut for a = a * 2
a /= 2 is a shortcut for a = a / 2
a %= 2 is a shortcut for a = a % 2
a &= 2 is a shortcut for a = a & 2
a |= 2 is a shortcut for a = a | 2
a ^= 2 is a shortcut for a = a ^ 2
a <<= 2 is a shortcut for a = a << 2
a >>= 2 is a shortcut for a = a >> 2
a >>>= 2 is a shortcut for a = a >>> 2
Transparency No. 1-11
Basic Java Syntax
The Conditional Operator
 syntax:
 BooleanExpr ? expr1 : expr2
 Ex:
 int max = (x > y) ? x : y;
 String name;
 name = (name != null)? name : “unknown”;
Transparency No. 1-12
Basic Java Syntax
Array and array Declaration (details deferred)
Syntax:
 arrayType arrayName[] ( = new arrayType[size] );
 arrayType[] arrayName ( = new arrayType[size] );
 arrayType[] arrayName = {initValue1, initValue2, ... initValueN};
 Ex:
 int[] a;
int[] c = {1,2,3};
 int[] b = new int[10]; // b[0] .. b[9]
initialized to 0.
 => c.length == 3 // true!
 => b[2] == 0 // true
 => a[2] == 0 // error!! no space assigned yet
 => a == null // true
Transparency No. 1-13
Basic Java Syntax
Array Example
class ArrayDeclaration {
public static void main(String args[]) {
// Demonstration of 3 techniques for array declaration;
int
a[] = new int[10];
int[] b
= new int[10];
int[] c
= {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
//Like C/C++ arrays,Java arrays are indexed from 0
c[3] = 5;
out.println(c[3]);
b[4] = 0;
b[4]++;
out.println(b[4]);
out.println(b[5]);
} }
Transparency No. 1-14
Basic Java Syntax
The instanceof operator
 Check if an object (reference) is an instance of the
specified type.
 syntax:
 o instanceof type
 Examples:








“string” instanceof String // true
“” instanceof Object // true
new int[ ] {1} instanceof int[] // true
new int[ ] {1} instanceof byte[] // false
new int[ ]{1} instanceof Object // true
null instanceof Object // false
// use instanceof to check if its safe to cast.
if(object instanceof Point) Point p = (Point) object;
Transparency No. 1-15
Basic Java Syntax
Java Statements
 A statement is a single “command” that is executed
by the java interpreter.
 By default, the java interpreter run one statement
after another, in the order they are written.
 Like most PLs, many of the Java statements are
flow-control statements that alter the default order
of execution in well-defined ways.
 IfThenElse, Switch,
 WhileDo, DoWhile, For,…
Transparency No. 1-16
Basic Java Syntax






Java Statements summary
Statement purpose
expression sideEffect
(block)
compound group statements
empty
doNothing
labled
name a statement
variable
declare a variable
if
conditional
switch
conditional




while
loop
do
loop
for
simplified loop
enhanced for (1.5)


syntax
var=expr ;
expr++ ;
method() ;
new type() ;
{ statement1 … statementn } (n ≥ 0)
;
label: statement
[final] type name [=val [, name = val]*];
if (expr) statement [else statement]
switch(expr) {
[case expr: statements]*
[default: statements]
}
while (expr) statement
do statement while (expr);
for(init;test;increment) statement
for( Type var : arrayOfType) statement
Transparency No. 1-17
Basic Java Syntax
Java Statements summary (cont’d)





statement
break
continue
return
synchronized
 throw
 try
purpose
exit loop
start next loop
end method
critical section
throw exception
handle exception
syntax
break [label];
continue [label];
return [expr];
synchronized (expr)
{ statements}
throw expr;
try{ statements }
[ catch(type name) {statements}]*
[finally {statements} ]
 Note that some statements do not end with “;”.
Transparency No. 1-18
Basic Java Syntax
Expression Statements
 Any Java expressions with side effect can be used
as a statement simply by following it with a
semicolon.
 legal expression statements:
 assignment, increment/decrement
 method call, object creation.
Ex:





a = 1 ;
// assignment
x += 2;
// assignment with operation
i++;
// post increment
--c;
// pre-decrement
System.out.println(“Hello”); // method call
Transparency No. 1-19
Basic Java Syntax
Compound statement (block)
 A compound statement is any number of statements
grouped together within curly braces.
 can use compound statement anywhere a statement
is required by java syntax.
Ex:
for(int i = 0; i < 0; i++) {
a[i]++;
{ b--; c = a[i] ; };
}
 quiz: How many statements are there in the body of
the if statement ?
 Ans: 5!
Transparency No. 1-20
Basic Java Syntax
The Empty Statement
 written as a single semicolon.
 do nothing but occasionally useful.
Ex:
for(int i = 0; i < 10, a[i++]++)
// increment array element
;
// loop body is empty statement
Tip: Always append a semicolon after a statement if you do not
assure if it must end with semicolon.
Ex:
while (i > 0){
a[i]++;
// needed
b[i]++; } ;
// 1st‘;’ needed,2nd‘;’optional
i = 0;
Transparency No. 1-21
Basic Java Syntax
Labeled statements
 are statements prepended with an identifier (label)
and a colon.
 Labels are used by break and continue statements.
 note: java has no goto statement.
Ex:
outerLoop: for (int i = 0; i < a.length; i++) {
innerLoop: for(int j = 0; j < b.length; j++){
if(a[i] > b[j]) break; //= break innerLoop
else if ( a[i] = b[j]) continue outerLoop;
else break OuterLoop;
}
}
Transparency No. 1-22
Basic Java Syntax
Local Variable declaration statements
 In java, variables means local variables ( which are declared
within a method ), while global variables are called fields which
are declared within a class and outside of any method.)
 A local variable is
 a symbolic name for a location where a value can be stored
 defined within a method or a compound statement.
Transparency No. 1-23
Basic Java Syntax
Kinds of variables
1. Fields: // declared directly within class
1. class variable
2. instance variable
2. Array components
3. Parameters
1. Method parameters
2. Constructor parameters
3. exception-handler parameter
4. Local variables
 are variable declared within the body of a method or
initialization block.
Transparency No. 1-24
Basic Java Syntax
Example of variables
class Point {
static int numPoints;
// numPoints is a class variable
int[] w = new int[10];
// w[0]~w[9] are array components
int x, y;
// x and y are instance variables
{ int k = 10;
// k and i are local variables
for(int i = 0; i < w.length; i++) w[i] = k;
}
int setX(int x) {
// x is a method parameter
try{ int oldx = this.x;
// oldx is a local variable
this.x = x;
} catch(Exception e) {
// e is an catch-handler parameter
e.printStackTrace(); }
return oldx;
}
}
Transparency No. 1-25
Basic Java Syntax
(Local) Variable Declaration




[final] Type Name ;
[final] Type Name = initExpr ;
[final] Type Name [= initExpr [, name [=initExpr] ]* ;
Note:
 InitExpr is a runTime Expression instead of limited to compile-time constants.
 Final variable is readOnly and cannot be assigned new value once initialized.
Ex:








int counter; // Syntax 1: no default value assigned!
String s;
int i = 0;
// syntax 2
String s=readLine();// s’value is unknown when compiling
int[] data = {x+1, x+2, x+3}; // array initializer
int x, y = 1, z;
// x,z has no value yet!
float x = 1.0; y;
String q = “a good question”, ans,
Transparency No. 1-26
Basic Java Syntax
Final variables and default initial values
 Variables declared with the “final” modifier are final variables.
 Final variable is readOnly/writeOnce and cannot be assigned
new value once initialized.
 Ex:





final int
x = x+1;
final int
y = 10; y
final int
x = 10;
// compile error!
y;
// y has no value yet!
++;
// y = 10 ok! y++ error!
z = 10
// ok!
Transparency No. 1-27
Basic Java Syntax
Initial value of a (global) variable declared without initializer:
 Initial value of a variable declared without initializer:
 Integer types (byte,short,char,int) => 0 or \u0000
 floating-point type(float, double)
=> 0.0
 Reference(Object) Type (class,interface,array) => null
 Note:
 Applicable to global variables (i.e, fileds) only.
 NOT applicable to local variables!!
 Local variable has no value if befor it is assigned a value
explicitely.
 Ex:
 class A { int x ; // ok! the default x value of every A
instance is 0. }
 => new A().x == 0 // true
Transparency No. 1-28
Basic Java Syntax
Notes about (local) variable declarations
 must be declared before use.
 must be written before reading. I.e., no default value mechanism
 can occur anywhere in a method subject to the above constraint.
Ex: {
int i=0, j = 1;
i = i + j ;
int m = i + j;// ok, even some non-declaration
// statements proceed it.
k = i + j;
//error, declaration must occur
int k;
//before reference.
i = k + 1 // error since local k value not set yet!
}
Transparency No. 1-29
Basic Java Syntax
Scope rules of local variables
 The scope of a local variable declaration in a block is the rest of
the block in which the declaration appears, starting with its own
initializer.
 may not be shadowed(i.e., redeclared in its scope).
Ex:
{int i=/*i declaration starts here until last }*/
(i=2)*2, m = i+1;
{ int j = 1;
j’s scope
i = i + j;
// outer i is visible
i = k ;
// error, k not declared yet!
int i = 10;
// error, i is in scope of outer i
}
int k,j = 10 ; // j can be redeclared since it is not
in scope of any previous j.
}
Transparency No. 1-30
Basic Java Syntax
More Example:
public class test1
{ static int i = 10; // i behaves like global variable, can be
// shadowed by local var.
public static void main(String[] args){
int j = 3;
for(int i = 1, j =2 ; i < 5; i++) // error, j is shadowed
{ int args; // error, parameter cannot be shadowed
System.out.println("inner i=" + i);
}
System.out.println(“field i=“ + i ); // field i=10
{ int i = test1.i; // int i = i; => error! why ?
}
int i = 0; // ok! why ?
}}
Transparency No. 1-31
Basic Java Syntax
Flow of Control statements
 Selection
 IfThenElse
 Switch
 Iteration
 For
 whileDo
 DoWhile
Transparency No. 1-32
Basic Java Syntax
IfThenElse statement
1. if (expr) statement
2. if (expr) statement else statement
3. if (expr) statement
[ else if (expr) statement ]+
else statement
Note: Syntax 3 is a special case of syntax 2.
Ex:
 if (x > 0 ) x = - x;
 if(x>0&&y>0 || x<0&& y<0){z=x*y;};
//; must be removed
else z = -x * y; // ; is needed.
 if ( x > 0 ) y = -x
//error!must append;,y=-x not a statement
else y = x;
Transparency No. 1-33
Basic Java Syntax
Testing multiple conditions
if(n == 1) {
// do task 1
}
else•
if (n == 2){ // can’t use elseif !
// do task 2
}
else if( n == 3) {
// do task 3
}
else{
// do task 4
}
Transparency No. 1-34
Basic Java Syntax
Dangling Else
 when using nested if/else, make sure you know which else
goes with which if statement.
Ex:
 if(i == j)
if(j ==k)
println(“i == k”);
else println(“i != j); // wrong!
 Dangling else is by default attached to the innermost if.
 Correction: use block statement!!
 if(i == j) {
if(j ==k)
println(“i == k”);
}else
println(“i != j);
Transparency No. 1-35
Basic Java Syntax
Switch statement
switch (expr) {
case expr1:
[statements] [break;]
...
case exprN:
[statements] [break;]
default: [statements] [break;]
}
Notes:
1. same semantics as in C
2. Expr must be of integer type (byte, short, char or int; long, float and
double are not allowed).
3. expr1…exprN must be compile-time constant expression.
4. Duplicate cases with the same values not allowed.
5. Multiple defaults not allowed.
6. Default need not occur at the last position.
Transparency No. 1-36
Basic Java Syntax
Example1:
class Toomany {
static void howMany(int k) {
switch (k) {
case 1: out.print("one ");
case 2: out.print("too ");
default : // same as case 3
case 3: out.println("many");
}
}
public static void main(String[] args) {
howMany(3); // output: many
howMany(2); // output: too many
howMany(1); // output: one two many
howMany(6); // output: many
}}
Transparency No. 1-37
Basic Java Syntax
Example2:
class Toomany {
static void howMany(int k) {
switch (k) {
case 1: out.print("one "); break;
case 2: out.print("too "); break;
case 3: out.println("many");
break; // not needed!
}
}
public static void main(String[] args) {
howMany(1); // output: one
howMany(2); // output: too
howMany(3); // output: many
}
}
Transparency No. 1-38
Basic Java Syntax
Iteration
 while (booleanExpr)
statement
 for ([initialization];[booleanExpr]; [iteration])
statement
// basic for
 for (Type var : arrayOfType)
statement
// enhanced for: java 1.5 or above
 do statement
while (booleanExpr);
// semicolon is needed.
Transparency No. 1-39
Basic Java Syntax
Example
class Iteration {
public static void main(String args[]) {
for (int index = 1; index <= 10; index++)
{ out.println("for: index = " + index); }
int index = 1;
while (index <= 10)
{ out.println("while: index = " + index);
index++; }
index = 1;
do{
out.println("do while: index=" + index);
index++;
} while (index <= 10);
} }
Transparency No. 1-40
Basic Java Syntax
Example : Enhanced For
int sum(int[] a) {
int sum = 0;
for (int i : a) sum += i;
return sum;
}
… sum( new int[] {1,2,3,4,5} )
 15
Transparency No. 1-41
Basic Java Syntax
Some notes about for statement
for([initialization];[booleanExpr];[increments])
statement


initialization allows declaration of multi-local variables scoped
to the for-statement only.
can use comma to separate multiple variables declarations [in
a single] initialization and increment expressions.
Ex:
1. for(int i=2,j=i+10; i<10; i=i+1,j--)
2.
print(“i+j=“ + i*j ) ;
3. print(i) // error! i not declared !
Note: Replace Line 1 by: for(int i =0,int j = 10; i < 10; i++, j--) is
incorrect, even if ,int is changed to ;int.
Hence it is impossible to declare variables with different types in
initialization
Transparency No. 1-42
Basic Java Syntax
The break Statement
break [Identifier ];
 A break statement transfers control out of an enclosing statement.
 A break statement with no label attempts to end the innermost
enclosing switch, while, do, or for statement of the immediately
enclosing method or initializer block; this statement is called
the break target.
 A break statement with label Identifier attempts to end the
enclosing labeled statement that has the same Identifier as its
label.
 In this case, the break target need not be a while, do, for, or switch
statement.
Transparency No. 1-43
Basic Java Syntax
public Graph loseEdges(int i, int j) {
int n = edges.length; nt[][] newedges = new int[n][];
for (int k = 0; k < n; ++k) {
edgelist: { int z;
search:
{ if (k == i) {
for (z = 0; z < edges[k].length; ++z)
if (edges[k][z] == j) break search;
} else if (k == j) {
for (z = 0; z < edges[k].length; ++z)
if (edges[k][z] == i) break;
}
// No edge to be deleted; share this list.
newedges[k] = edges[k];
break edgelist;
} //search
// Copy the list, omitting the edge at position z.
int m = edges[k].length - 1;
int ne[] = new int[m];
System.arraycopy(edges[k], 0, ne, 0, z);
System.arraycopy(edges[k], z+1, ne, z, m-z);
newedges[k] = ne;
} //edgelist
}
return new Graph(newedges);
}
Transparency No. 1-44
Basic Java Syntax
One more example
public class Test
{ static int i = 10, k = 11;
public static void main(String[] args){
lab1 : {
int i = 0;
if (i > 0) break lab1; // ok
}
lab2 : {
int i = test1.i;
if (i > 0) break ; // error, no enclosing switch or loop
}
}}
Transparency No. 1-45
Basic Java Syntax
The continue statement
 A continue statement go to the end of the current iteration of a
loop and starts the next one.
 can be used only within a loop (while, do or for loop).
 Without label => cause the innermost loop to start a new
iteration.
 With label => cause the named target loop to start a new
iteration
Ex:
for(int i =0; i < 10; i++){
if( i % 2 == 0) continue;
goto here!
print(i);
}.
// only odd numbers are printed.
Transparency No. 1-46
Basic Java Syntax
public Graph loseEdges(int i, int j) {
int n = edges.length;
int[][] newedges = new int[n][];
edgelists: for (int k = 0; k < n; ++k) {
int z;
search: {
if (k == i) {
...
} else if (k == j) {
...
}
newedges[k] = edges[k];
continue edgelists; // edgelsits not needed here!
} // search
...
} // edgelists
return new Graph(newedges);
Transparency No. 1-47
}
Basic Java Syntax
Return statement
 syntax:
 return;
 return expr;
 stops execution of the current method or construction
with/without value returned.
 Syntax 1 used in void method or constructor without returning
value.
 syntax 2 used in non-void method., which need return a value.
Ex:
 double square(double x){ return x * x; }
 void printsqrt(double x){
if (x < 0 ) return; // ‘return null’ => error
System.out.println(Math.sqrt(x)); // return implicitly
}
Transparency No. 1-48
Basic Java Syntax
The synchronized statement (skipped)
 Synchronized statement:
form: synchronized( ObjExpr ) statement
Semantics:
1. wait until ObjExpr is unlocked
2. try to lock the ObjExpr and perform the statement.
3. release the lock on ObjExpr (so that other thread waiting
for the ObjExpr has chance to contrinue)
 detail deferred until Multi-thread programming
 Note: ObjExpr must evaluate to an object.





Transparency No. 1-49
Basic Java Syntax
Example of the synchronized statement (skipped!!)
public static void SortArray(int[] a) {
// sort the array a. this is synchronized so that other thread cannot
// change elements of the array while we are sorting it.
synchronized(a) {
// do the actual sorting here
…
} }
Note: The synchronized keyword is more frequently used as a
method modifier serving the same role as in synchronized
statement.
Transparency No. 1-50
Basic Java Syntax
Synchronized method (skipped!!)
1. the instance method:
synchronized type method1(…) { statements }
is equivalent to the following:
type method1(…) { synchronized(this) { statements} }
2. the class method in class CLS:
class CLS { …
synchronized static type method2(…) { statements }
}
is equivalent to the following:
class Cls { …
static type method2(…) { synchronized(Cls.class) { statements}}
}
Transparency No. 1-51
Basic Java Syntax
The throw statement (skipped!!)
 An exception is a signal (or simply object) indicating
that some sort of exceptional condition or error has
occurred.
 To throw an exception is to signal an exception
condition.
 To catch an exception is to handle it – to take whatever
actions required to recover from it.
 detail deferred
Form: throw ExceptionExpr;
Transparency No. 1-52
Basic Java Syntax
The try-catch-finally statement (skipped!!)
 Java’s exception handling mechanism.
try { statements1… }
catch (Exception1 e1) {
// statements for handling exceptions (1) of type Exception1 or
// its subclass and (2) are thrown from statements1
…}
…
catch(EsxceptionN, eN) // N >=0
// statements for handling exceptions (1) of type ExceptionN or
// its subclass ,(2)thrown from statements1 and (3) are not caught
// by previous catches.
… } // see next slide
Transparency No. 1-53
Basic Java Syntax
The try-catch-finally statement (continued)
finally { statements2 // finally clause is optional
// this block contains statements that are always executed after
leaving the try-block, regardless of whether we leave if:
// 1. normally, after reaching the bottom of the block
// 2. because of break, continue or return
// 3. with an exception handled by a catch handler
//4. with an exception uncaught.
// if terminated by executing System.exit() in the try-block,
// finally clause will not executed.
…
}
Transparency No. 1-54
Basic Java Syntax
Java source file and java class definitions
1. Source file is the compilation unit of Java.
2. Each source file may contain multiple java class (and/or
interface) definitions.

But each file may contain at most one public class (or interface).
3. If a source file contains a public class of name Point, then this
source file must be named as Point.java
4. If a source file contains a class named A then javac will
produce a single file named A.class for the generated java byte
code for such class.
Transparency No. 1-55
Basic Java Syntax
Example:
 The source file contains the following code :
package a.b.c;
// this file and its contained classes are in package a.b.c.
import java.lang.*; // import statement let you use simple class name like
// System in place of its fully quantified name: java.lang.System
import java.io.IOException;
…
class A { … }
public class B { … }
interface C { … }
 must be named B.java
 After successful compilation, javac will produce three byte
code files named A.class, B.class and C.class, respectivley.
Transparency No. 1-56
Basic Java Syntax
Array types revisited
 Declare variables of array type:






1. byte b; // b == 0 or undefined now!
2. byte[] ArrayofBytes // == null or undefined
// byte[] is an array type: array of bytes
3. byte[][] ArrayofArrayOfBytes // byte[][] is an array type:
// array of byte[]
4. Points[] points ; // Point[] is an array of Point objects
 Equivalent declarations (not recommended!!):




1. byte b;
2. byte[][] ArrayofAarrayOfBytes;
3. byte[] ArrayofArrayOfBytes[];
3. byte ArrayOfArrayOfBytes [][];
Transparency No. 1-57
Basic Java Syntax
Creating Arrays
 Form: new Type[size] ;
Ex:
byte[] buffer = new byte[1024];
String[] lines = new String[50];
int size = getFromInput();
String[] lines2 = new String[size];
// note: size is not known until runtime.
 Default values for array elements:
 interger => 0; float => 0.0
 boolean => false
 reference => null.
Transparency No. 1-58
Basic Java Syntax
Using Arrays
 By index
String[] resp = new String[2];
resp[0] = “yes”; // Java array is 0-based
resp[1] = “no”
resp[ resp.length ] = “fault” ;
// length is a public field of array object containing
// the number of elements of the array.
// this will raise ArrayIndexOutOfBoundsException
Transparency No. 1-59
Basic Java Syntax
Initialization of large regular arrays
use initialization block:
class A { …
int [] a = new int[size] ;
int total = 0;
{ int len = a.length; // len is local var
for(int i = 0; i < len; ) a[i] = i++;
} // end of first instance initialization block.
…
}
Transparency No. 1-60
Basic Java Syntax
Array Literals
char[] passwd = null ;
int[] pOfTwo = new int[] {1,2,4,8,16}; // new int[] may be skipped!!
// pOfTwo.length == 5
// anonymous arrays (literal):
String resp = askQ( “Do you want to quit?”,
new String[] {“yes”, “no”} );
double d = computeAreaOfTriangle(
new Point[] { new Point(1,2),
new Point(3,4),
new Point(5,6) }
);
Transparency No. 1-61
Basic Java Syntax
How javac deal with array literals
int [] pn = {6, 8};
is compiled into code equivalent to:
int[] pn= new int[2];
{ pn[0]= 6; pn[1] = 8; }
 Hence it is not a good idea to include a large amount
of data in an array literal.
 You should store them in an external file and read them at
runtime.
 Array literals need not be constants.
Point[] points = {c1.getPoint(), c2.getPoint() }
Transparency No. 1-62
Basic Java Syntax
Multidimensional Arrays
 Consider the declaration:
int [][] prod = new int[5][10]; // prod has length 5!!
 Some PL may produce a block of 100 int values
 Java does not work this way: Instead java treat it like the
following code:
int prod[][] = new int[5][]; // neither int[][10] nor int[][5] !
{ for(int i = 0; i < 5; i++ )
prod[i] = new int[10] ; // create 5 subarrays of 10 elements
}
Transparency No. 1-63
Basic Java Syntax
prod
prod[0]
prod[1]
prod[2]
prod[3]
prod[4]
prod[4][0]
prod[0][0]
prod[0][9]
Transparency No. 1-64
Basic Java Syntax
More examples
 float [][][] g = new float[10][20][30]; // 6000 float spaces allocated
 float [][][] g = new float [10][][]; // 10 null array references
allocated
 > int x[][] = new int[10][];
 > x[2]  null !!
// an array of length 10 of type int[][]
 float [][][] g = new float [10][20][];
 float [][][] g = new float [10][][30]; // error
 float [][][] g = new float [][20][30]; //error
Transparency No. 1-65
Basic Java Syntax
Use array literal to create non-rectangular array:
 int [][] prod = { {0,0,0}, {1,1,1}, {2,2,,2}, {3,3,3}};
// prod.length == 4; prod[1].length = 3.
 int [][] p2 = { {0}, {1,1}, {2,2,2},{3,3,3,3}};
// p2.length = 4; p2[i].length = i + 1.
 Use for loop to create a large triangular table
int[][] p = new int[12][]; // an array of 12 int[]
{
for (int r = 0; r < 12; r++) {
p[r] = new int[r+1];
for(int c = 0; c < r+1; c++)
p[r][c] = r * c;
}}
Transparency No. 1-66
Basic Java Syntax
Importing classes and packages
An import directive of the form:
import a.b.C;
appearing after the package directive allows you to refer to class C
by its simple name C instead of its full name a.b.C.
 Similarly, the directive:
import a.b.*;
allows you to refer to all classes C of package a.b by simple name
C instead of a.b.C.
 The package java.lang are used very frequently and hence by
default is imported to all file:
import java.lang.*;
Transparency No. 1-67
Basic Java Syntax
Importing static methods and fields ( new in java 5.0)
An import directive of the form:
import static a.b.C.m1; or
import static a.b.C.field2;
allows you to refer to class method C.m1() by method name m1()
or static variable C.field2 by fields2 without using the class
name C or a.b.C.
 Similarly, the directive:
import static a.b.C.*;
allows you to refer to all static members (fields or methods) of
class C without using the class name C or a.b.C.
ex: import static java.lang.System.*;
==> out.println(“abc”); in.read(); exit(0) ; // all are legal!!
Transparency No. 1-68