How are Java objects stored in memory?

1.3

In Java, all objects are dynamically allocated on Heap. This is different from C++ where objects can be allocated memory either on Stack or on Heap. In C++, when we allocate abject using new(), the abject is allocated on Heap, otherwise on Stack if not global or static.

In Java, when we only declare a variable of a class type, only a reference is created (memory is not allocated for the object). To allocate memory to an object, we must use new(). So the object is always allocated memory on heap (See this for more details).

For example, following program fails in compilation. Compiler gives error “Error here because t is not initialed”.

class Test {
    // class contents
    void show() {
        System.out.println("Test::show() called");
    }
}

public class Main {
    public static void main(String[] args) {
        Test t; 
        t.show(); // Error here because t is not initialed
    }
}

Allocating memory using new() makes above program work.

class Test {
    // class contents
    void show() {
        System.out.println("Test::show() called");
    }
}
 
public class Main {
    public static void main(String[] args) {
        Test t = new Test(); //all objects are dynamically allocated
        t.show(); // No error 
    }
}

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