Java if and else

Java - General
 Java is:
– platform independent programming
language
– similar to C++ in syntax
– similar to Smalltalk in mental paradigm
 Pros: also ubiquitous to net
 Cons: interpreted, and still under
development (moving target)

How it works…!
Compile-time Environment Compile-time Environment

Class
Loader Java
Class
Bytecode Libraries
Java Verifier
Source
(.java)

Just in
Java Java
Time
Bytecodes Interpreter Java
Compiler
move locally Virtual
or through machine
Java network
Compiler
Runtime System

Java Operating System
Bytecode
(.class )
Hardware

Primitive Types and Variables
 boolean, char, byte, short, int, long, float, double etc.
 These basic (or primitive) types are the only types
that are not objects (due to performance issues).
 This means that you don’t use the new operator to
create a primitive variable.
 Declaring primitive variables:
float initVal;
int retVal, index = 2;
double gamma = 1.2, brightness
boolean valueOk = false;

Initialisation
 If no value is assigned prior to use, then the
compiler will give an error
 Java sets primitive variables to zero or false
in the case of a boolean variable
 All object references are initially set to null
 An array of anything is an object
– Set to null on declaration
– Elements to zero false or null on creation

Declarations
int index = 1.2; // compiler error
boolean retOk = 1; // compiler error
double fiveFourths = 5 / 4; // no error!
float ratio = 5.8f; // correct
double fiveFourths = 5.0 / 4.0; // correct
 1.2f is a float value accurate to 7 decimal places.
 1.2 is a double value accurate to 15 decimal places.

Assignment
 All Java assignments are right associative
int a = 1, b = 2, c = 5
a=b=c
System.out.print(
“a= “ + a + “b= “ + b + “c= “ + c)

 What is the value of a, b & c
 Done right to left: a = (b = c);

Basic Mathematical Operators
 * / % + - are the mathematical operators
 * / % have a higher precedence than + or -
double myVal = a + b % d – c * d / b;
 Is the same as:
double myVal = (a + (b % d)) –
((c * d) / b);

If – The Conditional Statement
 The if statement evaluates an expression and if that
evaluation is true then the specified action is taken
if ( x < 10 ) x = 10;
 If the value of x is less than 10, make x equal to 10
 It could have been written:
if ( x < 10 )
x = 10;
 Or, alternatively:
if ( x < 10 ) { x = 10; }

Relational Operators
== Equal (careful)
!= Not equal
>= Greater than or equal
<= Less than or equal
> Greater than
< Less than

If… else
 The if … else statement evaluates an expression and
performs one action if that evaluation is true or a
different action if it is false.
if (x != oldx) {
System.out.print(“x was changed”);
}
else {
System.out.print(“x is unchanged”);
}

Nested if … else
if ( myVal > 100 ) {
if ( remainderOn == true) {
myVal = mVal % 100;
}
else {
myVal = myVal / 100.0;
}
}
else
{
System.out.print(“myVal is in range”);
}

else if
 Useful for choosing between alternatives:
if ( n == 1 ) {
// execute code block #1
}
else if ( j == 2 ) {
}
else {
// if all previous tests have failed,
execute code block #3
}

A Warning…
WRONG! CORRECT!
if( i == j ) if( i == j ) {
if ( j == k )
if ( j == k )
System.out.print(
System.out.print(
“i equals k”);
“i equals k”);
}
else
else
System.out.print(
“i is not equal
System.out.print(“
to j”);
i is not equal to
j”); // Correct!

The switch Statement
switch ( n ) {
case 1:
break;
case 2:
break;
default:
// if all previous tests fail then
//execute code block #4
break;
}

The for loop
 Loop n times
for ( i = 0; i < n; n++ ) {
// this code body will execute n times
// ifrom 0 to n-1
}
 Nested for:
for ( j = 0; j < 10; j++ ) {
for ( i = 0; i < 20; i++ ){
// this code body will execute 200 times
}
}

while loops
while(response == 1) {
System.out.print( “ID =” +
userID[n]);
n++;
response = readInt( “Enter “);
}
What is the minimum number of times the loop
is executed?
What is the maximum number of times?

Java if and else

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Java if and else