Heap memory in Java

Introduction

Heap memory is where Java stores objects and data created during a program's run. It helps manage memory so your program can use space efficiently.

When you create new objects using the <code>new</code> keyword.

When you want to store data that needs to live beyond a single method call.

When your program needs to manage memory dynamically as it runs.

When you want to understand how Java manages memory to avoid errors like memory leaks.

Syntax

Java

public class Example {
    public static void main(String[] args) {
        // Creating an object stored in heap memory
        MyObject myObject = new MyObject();
    }
}

class MyObject {
    int value;
    // Object fields and methods
}

Objects created with new are stored in heap memory.

Primitive variables like int inside methods are stored in stack memory, but objects live in heap.

Examples

This creates a new object in heap memory and obj holds a reference to it.

Java

MyObject obj = new MyObject();

This stores a primitive value on the stack, not in heap memory.

Java

int number = 5;

Here, obj points to nothing, so no heap memory is allocated.

Java

MyObject obj = null; // No object created, no heap memory used

Sample Program

This program creates two Person objects stored in heap memory. It shows how references point to objects and how setting a reference to null means the object can be cleaned up by Java's garbage collector.

Java

public class HeapMemoryDemo {
    public static void main(String[] args) {
        System.out.println("Start of program");

        // Create first object in heap
        Person personOne = new Person("Alice", 30);
        System.out.println("Person One: " + personOne);

        // Create second object in heap
        Person personTwo = new Person("Bob", 25);
        System.out.println("Person Two: " + personTwo);

        // Nullify reference to personOne
        personOne = null;
        System.out.println("Person One reference set to null");

        System.out.println("End of program");
    }
}

class Person {
    String name;
    int age;

    Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String toString() {
        return name + ", age " + age;
    }
}

Important Notes

Time complexity: Creating objects in heap is generally fast but depends on JVM implementation.

Space complexity: Heap size limits how many objects you can create; too many objects can cause OutOfMemoryError.

Common mistake: Confusing stack and heap memory. Variables hold references on stack, but objects live in heap.

Use heap memory for objects that need to live beyond method calls. Use stack for temporary primitive variables.

Summary

Heap memory stores all objects created with new in Java.

References to objects live on the stack, but the actual objects live in heap.

Java automatically manages heap memory with garbage collection to free unused objects.

Practice

1. Which of the following best describes heap memory in Java?

easy

A. It stores method call information and local variables.

B. It stores primitive variables like int and double.

C. It stores all objects created using new keyword.

D. It stores the Java bytecode instructions.

Solution

1. Step 1: Understand what heap memory stores

   Heap memory is used to store objects created dynamically using the new keyword in Java.

2. Step 2: Differentiate from other memory areas

   Primitive variables and method calls are stored on the stack, not heap. Bytecode is stored in method area, not heap.

3. Final Answer:

   It stores all objects created using new keyword. -> Option C

4. Quick Check:

   Heap memory = objects from new [OK]

Hint: Heap = objects created with new keyword [OK]

Common Mistakes:

• Confusing stack with heap memory
• Thinking primitives are stored in heap
• Mixing method area with heap

2. Which of the following Java statements correctly creates an object stored in heap memory?

easy

A. String s = new String("hello");

B. int x = 5;

C. double y = 3.14;

D. boolean flag = true;

Solution

1. Step 1: Identify object creation syntax

   Objects in Java are created using the new keyword, which allocates memory on the heap.

2. Step 2: Check each option

   String s = new String("hello"); uses new String("hello"), which creates a String object on the heap. Other options declare primitives stored on the stack.

3. Final Answer:

   String s = new String("hello"); -> Option A

4. Quick Check:

   Object creation uses new keyword [OK]

Hint: Look for 'new' keyword to spot heap allocation [OK]

Common Mistakes:

• Assuming primitives use heap
• Confusing string literals with new String()
• Ignoring that only new creates heap objects

3. What will be the output of the following Java code?

class Test {
  public static void main(String[] args) {
    String a = new String("abc");
    String b = a;
    a = null;
    System.out.println(b);
  }
}

medium

A. null

B. Runtime exception

C. Compilation error

D. abc

Solution

1. Step 1: Understand object references and heap

   Variable a points to a String object "abc" in heap. Then b is assigned the same reference.

2. Step 2: Analyze effect of setting a to null

   Setting a = null removes one reference, but b still points to the object, so it remains accessible.

3. Step 3: Print b

   Printing b outputs the string "abc".

4. Final Answer:

   abc -> Option D

5. Quick Check:

   Reference b still points to "abc" [OK]

Hint: Nulling one reference doesn't delete object if others exist [OK]

Common Mistakes:

• Assuming object is deleted when one reference is null
• Confusing references with objects
• Expecting null output incorrectly

4. Identify the error in the following Java code related to heap memory usage:

public class Demo {
  public static void main(String[] args) {
    int[] arr = new int[5];
    arr = null;
    System.out.println(arr.length);
  }
}

medium

A. No error, prints 5

B. NullPointerException at runtime

C. Compilation error due to null assignment

D. Array size becomes zero

Solution

1. Step 1: Understand array creation and null assignment

   The array is created on heap with size 5, then arr reference is set to null.

2. Step 2: Analyze printing arr.length

   Since arr is null, accessing arr.length causes a NullPointerException at runtime.

3. Final Answer:

   NullPointerException at runtime -> Option B

4. Quick Check:

   Accessing member on null reference throws exception [OK]

Hint: Accessing null reference causes NullPointerException [OK]

Common Mistakes:

• Thinking null assignment causes compile error
• Expecting array length to print after null
• Confusing null with empty array

5. Consider this Java code snippet:

class Node {
  int value;
  Node next;
  Node(int val) { value = val; }
}

public class Test {
  public static void main(String[] args) {
    Node n1 = new Node(1);
    Node n2 = new Node(2);
    n1.next = n2;
    n2.next = n1;
    n1 = null;
    n2 = null;
    // What happens to the Node objects in heap?
  }
}

What happens to the two Node objects in heap after n1 and n2 are set to null?

hard

A. Both objects become unreachable and eligible for garbage collection.

B. Objects remain in heap because they reference each other, preventing garbage collection.

C. Only n1 object is collected, n2 remains.

D. Objects cause a memory leak and crash the program.

Solution

1. Step 1: Analyze object references after null assignments

   Both n1 and n2 references are set to null, so no external references point to these objects.

2. Step 2: Understand garbage collection with cyclic references

   Even though the two objects reference each other, Java's garbage collector detects unreachable cycles and frees them.

3. Final Answer:

   Both objects become unreachable and eligible for garbage collection. -> Option A

4. Quick Check:

   Unreachable objects, even cyclic, are collected [OK]

Hint: Java GC collects unreachable cyclic objects too [OK]

Common Mistakes:

• Thinking cyclic references prevent garbage collection
• Assuming memory leak without external references
• Confusing references with reachability