Operators

Compound Operators

• +=, -=, *= and /=
• Makes code compact
• Operator precedence for compound assignments : The expression on the right side of the = will always be evaluated first!

int x = 2;

x *= 3 + 5;

//This is equivalent to x = x * (3 + 5) and NOT x = (x*2) + 5.

Relational Operators

• Can be applied to ANY combination of integers, floats or chars.
• int's can be compared to doubles
• char's can be compared to int's

                int i = 4;
		double pi = 3.14;
		
		if( i > pi) {
			System.out.println("Greater than pi");
		}
		
		char c = 'C';
		
		if( c > 66) {
			System.out.println("C > B");
		}

Equality Operators

• '==' and '!=' are used to test for equality.
• Can't compare incompatible types.
• For e.g can't compare an int and a boolean or a Thread and a String
• Objects in the same hierarchy can be checked however.

Object o = new Object();
String s = new String("a");
Integer i = new Integer(42);
		
if(o == s) { //String IS-A object
 System.out.println("o==s");
} 
		
if (s == i) { // Compiler error
}

• For enums, == is equivalent to equals(), because there's no way to create additional enum constants after the declaration.

instanceof

• The instanceof (all small letters) is used for object variables only.
• The first operand must be an object instance and the second operator a class name.
• Instanceof can be used to compare operands which are in the same class hierarchy, However this restriction does not apply to interfaces !

class GrandFather {
}

class Father extends GrandFather {
}

class Son extends Father {
}

public class InOf {

	public static void main(String[] args) {
		Father f = new Father();
		GrandFather gf = new GrandFather();
		Son s = new Son();
		
		if (f instanceof GrandFather) { //OK
		}
		
		if (f instanceof Son) { //OK
		}
		
		if (f instanceof Thread) { //ILLEGAL COMPARE : NOT OK
		}

                if (f instanceof Runnable) { //OK TO COMPARE
                }
		
	}
}

• If a class's superclass implements an interface then the class will pass the instanceof test for the interface

interface Person {
}

class GrandFather implements Person {
}

class Father extends GrandFather {
}

class Son extends Father {
}

public class InOf {

	public static void main(String[] args) {
		
		Son s = new Son();
		
		if (s instanceof Person) {
			System.out.println("Son is a Person"); //Will print Son is a Person
		}
		
	}

}

• IT IS LEGAL to use null reference in an instanceof check. It will always return false.
• Can also be used for arrays

Integer[] ia = new Integer[20];
		
if(ia instanceof Number[]) {
   System.out.println("Integer[] IS-A Number[]"); //Prints this 
}

Arithmetic Operators

• +, -, *, /, % are the arithmetic operators.
• *, /, % have a higher precedence than the + and - operators.

String concatenation operator

• '+' is the only "overloaded" operator in java and can be used to concatenate two strings together.
• WATCH OUT : When integers are concatenated with Strings :
• Evaluation runs from left to right
• If both operands are integers, then addition will take place.
• If one operand is a string, concatenation will happen.

System.out.println(1 + 2 + " abc"); // 3 abc
System.out.println(1 + " abc " + 2); // 1 abc 2
System.out.println(1 + 2 + " abc " + 3); // 3 abc 3
System.out.println(1 + 2 + " abc " + 3 + 4); // 3 abc 34

Conditional Operator

• Is a ternary operator (three operands)
• Syntax:
• x = (boolean expression) ? value to assign if bool expr is true : value to assign if bool expr is false.
• value should match with type of x

  int c  = (new Random().nextInt(10) > 5) ? 1 : 0;

Logical Operator

• Six logical operators - &, |, ^, !, &&, !!
• & , |, ^ are also bitwise operators - they compare variables bit by bit and return a value whose bits have been set according to the comparison made.

int i = 3;
int j = 5;

int k = i & j; //k = 1;