Difference between revisions of "Operators"
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* The '''instanceof''' (all small letters) is used for object variables only. | * 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. | * 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 ! | + | * Instanceof can be used to compare operands which are in the same class hierarchy, However this restriction '''does not apply to interfaces''' ! |
<syntaxhighlight lang="java5"> | <syntaxhighlight lang="java5"> | ||
Line 105: | Line 105: | ||
</syntaxhighlight> | </syntaxhighlight> | ||
+ | * If a class's superclass implements an interface then the class will pass the instanceof test for the interface | ||
+ | |||
+ | <syntaxhighlight lang="java5"> | ||
+ | |||
+ | 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 | ||
+ | } | ||
+ | |||
+ | } | ||
+ | |||
+ | } | ||
+ | |||
+ | </syntaxhighlight> | ||
[[Category:OCPJP]] | [[Category:OCPJP]] |
Revision as of 04:00, 31 August 2011
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
}
}
}