Core Java Programming Language (JSE) : Chapter IV - Expressions and Flow Control

Java Programming Language SE – 6
Module 4 : Expressions and Flow Control

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Objectives
● Distinguish between instance and local variables
● Describe how to initialize instance variables
● Identify and correct a Possible reference before
assignment compiler error
● Recognize, describe, and use Java software operators
● Distinguish between legal and illegal assignments of
primitive types

Objectives
● Identify boolean expressions and their requirements in control
constructs
● Recognize assignment compatibility and required casts in
fundamental types
● Use if, switch, for, while, and do constructions and the labelled
forms of break and continue as flow control structures in a program

Relevance
● What types of variables are useful to programmers?
● Can multiple classes have variables with the same name and, if so,
what is their scope?
● What types of control structures are used in other languages? What
methods do these languages use to control flow?

Variables and Scope
Local variables are:
● Variables that are defined inside a method and are called local,
automatic, temporary, or stack variables
● Variables that are created when the method is executed are
destroyed when the method is exited
Variable initialization comprises the following:
● Local variables require explicit initialization.
● Instance variables are initialized automatically.

Variable Initialization

Initialization Before
Use Principle
The compiler will verify that local variables have been initialized
before used.
int x=8;
int y;
int z;
z=x+y;

Operator Precedence

Logical Operators
● The boolean operators are:
– ! – NOT
– | – OR
– & – AND
– ^ – XOR
● The short-circuit boolean operators are:
– && – AND
– || – OR

Logical Operators
You can use these operators as follows:
MyDate d = reservation.getDepartureDate();
if ( (d != null) && (d.day > 31) {
// do something with d
}

Bitwise Logical Operators
● The integer bitwise operators are:
– ~ – Complement
– ^ – XOR
– & – AND
– | – OR

Bitwise Logical Operators:
Example

Right-Shift Operators >>
and >>>
● Arithmetic or signed right shift ( >> ) operator:
● Examples are:
– 128 >> 1 returns 128/2 1 = 64
– 256 >> 4 returns 256/2 4 = 16
– -256 >> 4 returns -256/2 4 = -16
● The sign bit is copied during the shift.
● Logical or unsigned right-shift ( >>> ) operator:
– This operator is used for bit patterns.
– The sign bit is not copied during the shift.

Left-Shift Operator <<
● Left-shift ( << ) operator works as follows:
– 128 << 1 returns 128 * 2 1 = 256
– 16 << 2 returns 16 * 2 2 = 64

Shift Operator Examples

String Concatenation With +
● The + operator works as follows:
– Performs String concatenation
– Produces a new String:
String salutation = "Dr.";
String name = "Pete" + " " + "Seymour";
String title = salutation + " " + name;

Casting
● If information might be lost in an assignment, the programmer must
confirm the assignment with a cast.
● The assignment between long and int requires an explicit cast.
long bigValue = 99L;
int squashed = bigValue;// Wrong, needs a cast
int squashed = (int) bigValue; // OK
int squashed = 99L;// Wrong, needs a cast
int squashed = (int) 99L;// OK, but...
int squashed = 99; // default integer literal

Promotion and Casting
of Expressions
● Variables are promoted automatically to a longer form (such as int to
long).
● Expression is assignment-compatible if the variable type is at least as
large
long bigval = 6;// 6 is an int type, OK
int smallval = 99L; // 99L is a long, illegal
double z = 12.414F;// 12.414F is float, OK
float z1 = 12.414; // 12.414 is double, illegal

Simple if, else Statements
● The if statement syntax:
if ( <boolean_expression> )
<statement_or_block>
● Example:
if ( x < 10 )
System.out.println("Are you finished yet?");
or (recommended):
if ( x < 10 ) {
}

Complex if, else Statements
● The if-else statement syntax:
else
● Example:
if ( x < 10 ) {
} else {
System.out.println("Keep working...");
}

Complex if, else Statements
● The if-else-if statement syntax:
else if ( <boolean_expression> )

if-else-if statement: Example
● Example:
int count = getCount(); // a method defined in the class
if (count < 0) {
System.out.println("Error: count value is negative.");
} else if (count > getMaxCount()) {
System.out.println("Error: count value is too big.");
} else {
System.out.println("There will be " + count +
" people for lunch today.");
}

Switch Statements
The switch statement syntax:
switch ( <expression> ) {
case <constant1>:
<statement_or_block>*
[break;]
case <constant2>:
[break;]
default:
[break;]
}

Switch Statement Example
String carModel = ”STANDARD”;
switch ( carModel ) {
case DELUXE:
System.out.println(“DELUXE”);
break;
case STANDARD:
System.out.println(“Standard”);
break;
default:
System.out.println(“Default”);
}

Switch Statements
● Without the break statements, the execution falls through each
subsequent case clause.

For Loop
● The for loop syntax:
for ( <init_expr>; <test_expr>; <alter_expr> )

For Loop Example
for ( int i = 0; i < 10; i++ )
System.out.println(i + " squared is " + (i*i));
or (recommended):
for ( int i = 0; i < 10; i++ ) {
}

While Loop
The while loop syntax:
while ( <test_expr> )

Example:
int i = 0;
while ( i < 10 ) {
i++;
}
While Loop Example

The do/while Loop
● The do/while loop syntax:
do
while ( <test_expr> );

The do/while Loop:
Example
● Example:
int i = 0;
do {
i++;
} while ( i < 10 );

Special Loop Flow
Control
● The break [<label>]; command
● The continue [<label>]; command
● The <label> : <statement> command, where <statement> should be a
loop

The break Statement
do {
statement;
if ( condition ) {
break;
}
statement;
} while ( test_expr );

The continue Statement
do {
statement;
if ( condition ) {
continue;
}
statement;

Using break Statements
with Labels
outer:
do {
statement1;
do {
statement2;
if ( condition ) {
break outer;
}
statement3;
statement4;

Using continue Statements
with Labels
test:
do {
statement1;
do {
statement2;
if ( condition ) {
continue test;
}
statement3;
statement4;

Core Java Programming Language (JSE) : Chapter IV - Expressions and Flow Control

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Core Java Programming Language (JSE) : Chapter IV - Expressions and Flow Control