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
		}
		
	}

}