What is bridge method in Java?

Sometimes to support type-erasure use case of generics, Java compiler creates a synthetic method, which is called a bridge method, as part of the type erasure process.

 

What is type erasure?

Type Erasure is a process, where Java Compiler replaces each with its first bound if the type parameter is bounded, or Object if the type parameter is unbounded.

 

Box.java

public class Box<T> {

    private T data;

    public Box(T data) {
        this.data = data;
    }

    public T getData() { return data; }

}

 

In the above example, "T" is unbounded type. So Java Compiler replaces Every Occurrence of T with Object.

public class Box {

    private Object data;

    public Box(Object data) {
        this.data = data;
    }

    public Object getData() { return data; }

}

Consider one more Example

public class Box<T extends Integer> {

    private T data;

    public Box(T data) {
        this.data = data;
    }

    public T getData() { return data; }

}

After type Erasure, Type Parameter “T” is replaced with Integer.

public class Box{

    private Integer data;

    public Box(Integer data) {
        this.data = data;
    }

    public Integer getData() { return data; }

}

 

Bridge method

Sometimes to support type-erasure use case of generics, Java compiler creates a synthetic method, which is called a bridge method, as part of the type erasure process.

 

Let me explain it with an example.

 

Container.java

 

package com.sample.app;

public class Container<T> {

	public T data;

	public Container(T data) {
		this.data = data;
	}

	public void setData(T data) {
		this.data = data;
	}
}

 

IntegerContainer.java

package com.sample.app;

public class IntegerContainer extends Container<Integer> {

	public IntegerContainer(Integer data) {
		super(data);
	}

	public void setData(Integer data) {
		super.setData(data);
	}
}

 

Consider the following code.

IntegerContainer integerContainer = new IntegerContainer(5);

// A raw type - compiler throws an unchecked warning
Container container = integerContainer;

// Below statement throws a ClassCastException
container.setData("Hello");
Integer x = integerContainer.data;
After type erasure, this code becomes like below.

IntegerContainer integerContainer = new IntegerContainer(5);
Container container = (Container)integerContainer;         
container.setData("Hello");         
Integer x = (String)integerContainer.data;

 

As I said previously, in some situations, as part of type erasure process, Java compiler creates a synthetic method called bridge method.

 

After type erasure, the method signatures do not match; the Container.setData(T) method becomes Container.setData(Object). As a result, the IntegerContainer.setData(Integer) method does not override the Container.setData(Object) method.

 

To solve this problem and preserve the polymorphism of generic types after type erasure, the Java compiler generates a bridge method to ensure that subtyping works as expected.When you decompile the class file ‘IntegerContainer.class’, it contains below code snippet.

$javap -c com/sample/app/IntegerContainer.class 
Compiled from "IntegerContainer.java"
public class com.sample.app.IntegerContainer extends com.sample.app.Container<java.lang.Integer> {
  public com.sample.app.IntegerContainer(java.lang.Integer);
    Code:
       0: aload_0
       1: aload_1
       2: invokespecial #1                  // Method com/sample/app/Container."<init>":(Ljava/lang/Object;)V
       5: return

  public void setData(java.lang.Integer);
    Code:
       0: aload_0
       1: aload_1
       2: invokespecial #2                  // Method com/sample/app/Container.setData:(Ljava/lang/Object;)V
       5: return

  public void setData(java.lang.Object);
    Code:
       0: aload_0
       1: aload_1
       2: checkcast     #3                  // class java/lang/Integer
       5: invokevirtual #4                  // Method setData:(Ljava/lang/Integer;)V
       8: return
}

 

As you see above snippet, you can confirm that compiler generates a synthetic (bridge) method ‘public void setData(java.lang.Object);’,

 

The bridge method IntegerContainer.setData(object) delegates to the original IntegerContainer.setData(Integer) method. As a result, the container.setData("Hello"); statement calls the method IntegerContainer.setData(Object), and a ClassCastException is thrown because "Hello" can't be cast to Integer.

 

 

 

 

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