minte9
LearnRemember / JAVA



Objects 

 
/** 
 * To use an object you must declare a reference variable
 * JVM allocates space for the object
 * 
 * The reference variable is forever of that type
 * myDog = new Cat(); // will throw a type mismatch error
 */

package com.minte9.basics.objects;

public class Objects {
    public static void main(String[] args) {
    
        Dog myDog = new Dog();
        myDog.size = 40;
        myDog.bark();
    }
}

class Dog {
    int size;
        
    void bark() {
        System.out.println("Ham Ham"); // Ham Ham
    }
}

References 

 
/**
 * A reference to an object can be overridden
 * 
 * After a overrites b, the b reference to object 2 is destroyed
 * Eligible for Garbage Collection
 */

package com.minte9.basics.objects;

public class References {
    
    public static void main(String[] args) {
        
        Book a = new Book("A");
        Book b = new Book("B");
        System.out.printf("%s%s /", a, b); // AB

        Book c = b; // refc, objB
        System.out.printf("%s%s%s /", a, b, c); // ABB

        b = a; // refb, objA, objB destroyed
        System.out.printf("%s%s%s /", a, b, c); // AAB
    }
}

class Book {
    String name;   

    public Book(String name) { // contructor
        this.name = name;
    }
    public String toString() {
        return name;
    }
}



  Last update: 342 days ago




Questions and answers




Things that an object knows are named

  • a) methods
  • b) instance variables

Variable declaration tells JVM to

  • a) allocate a space for the reference
  • b) allocate space for object

In Java a reference type can be changed

  • a) true
  • b) false

JVM allocates a space for object

  • a) after using new myObject()
  • b) after using return

Overwriting an object reference

  • a) copy the reference
  • b) destroy the old reference


References





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