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1. Introduction

In this quick tutorial, we're going to take a look at the difference between instanceof, Class.isInstance, and Class.isAssignableFrom. We'll learn how to use each method and what the differences are between them.

2. Setup

Let's set up an interface and a couple of classes to use while we explore the instanceof, Class.isInstance, and Class.isAssignableFrom functionality.

First, let's define an interface:

public interface Shape {
}

Next, let's define a class that implements Shape:

public class Triangle implements Shape {
}

Now, we'll create a class that extends Triangle:

public class IsoscelesTriangle extends Triangle {
}

3. instanceof

The instanceof keyword is a binary operator, and we can use it to verify whether a certain object is an instance of a given type. Therefore, the result of the operation is either true or false. Additionally, the instanceof keyword is the most common and straightforward way to check if an object subtypes another type.

Let's use our classes with the instanceof operator:

Shape shape = new Triangle();
Triangle triangle = new Triangle();
IsoscelesTriangle isoscelesTriangle = new IsoscelesTriangle();
Shape nonspecificShape = null;

assertTrue(shape instanceof Shape);
assertTrue(triangle instanceof Shape);
assertTrue(isoscelesTriangle instanceof Shape);
assertFalse(nonspecificShape instanceof Shape);

assertTrue(shape instanceof Triangle);
assertTrue(triangle instanceof Triangle);
assertTrue(isoscelesTriangle instanceof Triangle);
assertFalse(nonspecificShape instanceof Triangle);

assertFalse(shape instanceof IsoscelesTriangle);
assertFalse(triangle instanceof IsoscelesTriangle);
assertTrue(isoscelesTriangle instanceof IsoscelesTriangle);
assertFalse(nonspecificShape instanceof IsoscelesTriangle);

With the above code snippet, we can see that the right-hand side type is more generic than the left-hand side object. More specifically, the instanceof operator will process null values to false.

4. Class.isInstance

The isInstance method on the Class class is equivalent to the instanceof operator. The isInstance method was introduced in Java 1.1 because it can be used dynamically. Generally, this method will return true if the argument isn't null and can be successfully cast to the reference type without raising a ClassCastException.

Let's look at how we can use the isInstance method with the interface and classes we defined:

Shape shape = new Triangle();
Triangle triangle = new Triangle();
IsoscelesTriangle isoscelesTriangle = new IsoscelesTriangle();
Triangle isoscelesTriangle2 = new IsoscelesTriangle();
Shape nonspecificShape = null;

assertTrue(Shape.class.isInstance(shape));
assertTrue(Shape.class.isInstance(triangle));
assertTrue(Shape.class.isInstance(isoscelesTriangle));
assertTrue(Shape.class.isInstance(isoscelesTriangle2));
assertFalse(Shape.class.isInstance(nonspecificShape));

assertTrue(Triangle.class.isInstance(shape));
assertTrue(Triangle.class.isInstance(triangle));
assertTrue(Triangle.class.isInstance(isoscelesTriangle));
assertTrue(Triangle.class.isInstance(isoscelesTriangle2));

assertFalse(IsoscelesTriangle.class.isInstance(shape));
assertFalse(IsoscelesTriangle.class.isInstance(triangle));
assertTrue(IsoscelesTriangle.class.isInstance(isoscelesTriangle));
assertTrue(IsoscelesTriangle.class.isInstance(isoscelesTriangle2));

As we can see, the right-hand side can be equivalent to or more specific than the left-hand side. In particular, providing null to the isInstance method returns false.

5. Class.isAssignableFrom

The Class.isAssignableFrom method will return true if the Class on the left-hand side of the statement is the same as or is a superclass or superinterface of the provided Class parameter.

Let's use our classes with the isAssignableFrom method:

Shape shape = new Triangle();
Triangle triangle = new Triangle();
IsoscelesTriangle isoscelesTriangle = new IsoscelesTriangle();
Triangle isoscelesTriangle2 = new IsoscelesTriangle();

assertFalse(shape.getClass().isAssignableFrom(Shape.class));
assertTrue(shape.getClass().isAssignableFrom(shape.getClass()));
assertTrue(shape.getClass().isAssignableFrom(triangle.getClass()));
assertTrue(shape.getClass().isAssignableFrom(isoscelesTriangle.getClass()));
assertTrue(shape.getClass().isAssignableFrom(isoscelesTriangle2.getClass()));

assertFalse(triangle.getClass().isAssignableFrom(Shape.class));
assertTrue(triangle.getClass().isAssignableFrom(shape.getClass()));
assertTrue(triangle.getClass().isAssignableFrom(triangle.getClass()));
assertTrue(triangle.getClass().isAssignableFrom(isoscelesTriangle.getClass()));
assertTrue(triangle.getClass().isAssignableFrom(isoscelesTriangle2.getClass()));

assertFalse(isoscelesTriangle.getClass().isAssignableFrom(Shape.class));
assertFalse(isoscelesTriangle.getClass().isAssignableFrom(shape.getClass()));
assertFalse(isoscelesTriangle.getClass().isAssignableFrom(triangle.getClass()));
assertTrue(isoscelesTriangle.getClass().isAssignableFrom(isoscelesTriangle.getClass()));
assertTrue(isoscelesTriangle.getClass().isAssignableFrom(isoscelesTriangle2.getClass()));

assertFalse(isoscelesTriangle2.getClass().isAssignableFrom(Shape.class));
assertFalse(isoscelesTriangle2.getClass().isAssignableFrom(shape.getClass()));
assertFalse(isoscelesTriangle2.getClass().isAssignableFrom(triangle.getClass()));
assertTrue(isoscelesTriangle2.getClass().isAssignableFrom(isoscelesTriangle.getClass()));
assertTrue(isoscelesTriangle2.getClass().isAssignableFrom(isoscelesTriangle2.getClass()));

As with the isInstance example, we can clearly see that the right-hand side must be either the same or more specific than the left-hand side. We can also see that we're never able to assign our Shape interface.

6. The Differences

Now that we've laid out a few detailed examples, let's go over some of the differences.

6.1. Semantical Differences

Superficially, instanceof is a Java Language keyword. In contrast, both isInstance and isAssignableFrom are native methods from Class type.

Semantically, we use them to verify the different relationships between two programming elements:

  • Two objects: we can test if the two objects are identical or equal.
  • An object and a type: we can check whether the object is an instance of the type. Obviously, both the instanceof keyword and the isInstance method belong to this category.
  • Two types: we can examine whether one type is compatible with another type, such as the isAssignableFrom method.

6.2. Usage Corner Case Differences

Firstly, they differ with a null value:

assertFalse(null instanceof Shape);
assertFalse(Shape.class.isInstance(null));
assertFalse(Shape.class.isAssignableFrom(null)); // NullPointerException

From the code snippet above, both instanceof and isInstance return false; however, the isAssignableFrom method throws NullPointerException.

Secondly, they differ with primitive types:

assertFalse(10 instanceof int); // illegal
assertFalse(int.class.isInstance(10));
assertTrue(Integer.class.isInstance(10));
assertTrue(int.class.isAssignableFrom(int.class));
assertFalse(float.class.isAssignableFrom(int.class));

As we can see, the instanceof keyword doesn't support primitive types. If we use the isInstance method with an int value, the Java compiler will transform the int value into an Integer object. So, the isInstance method supports primitive types but always returns false. When we use the isAssignableFrom method, the result depends on the exact type values.

Thirdly, they differ with class instance variables:

Shape shape = new Triangle();
Triangle triangle = new Triangle();
Class<?> clazz = shape.getClass();

assertFalse(triangle instanceof clazz); // illegal
assertTrue(clazz.isInstance(triangle));
assertTrue(clazz.isAssignableFrom(triangle.getClass()));

From the code snippet above, we realize that both isInstance and isAssignableFrom methods support the class instance variables, but the instanceof keyword doesn't.

6.3. Bytecode Differences

In a compiled class file, they utilize different opcodes:

  • The instanceof keyword corresponds to the instanceof opcode
  • Both the isInstance and isAssignableFrom methods will use the invokevirtual opcode

In the JVM Instruction Set, the instanceof opcode has a value of 193, and it has a two-byte operand:

instanceof
indexbyte1
indexbyte2

Then, the JVM will compute (indexbyte1 << 8) | indexbyte2 into an index. And this index points to the run-time constant pool of the current class. At the index, the run-time constant pool contains a symbolic reference to a CONSTANT_Class_info constant. And, this constant is exactly the value that the right-hand side of the instanceof keyword needs.

Furthermore, it also explains why the instanceof keyword can't use the class instance variable. This is because the instanceof opcode needs a constant type in the run-time constant pool, and we can't replace a constant type with a class instance variable.

And, where is the left-hand side object information of the instanceof keyword stored? It's on the top of the operand stack. So, the instanceof keyword requires an object on the operand stack and a constant type in the run-time constant pool.

In the JVM Instruction Set, the invokevirtual opcode has a value of 182, and it also has a two-byte operand:

invokevirtual
indexbyte1
indexbyte2

Similarly, the JVM will calculate (indexbyte1 << 8) | indexbyte2 into an index. At the index, the run-time constant pool holds a symbolic reference to a CONSTANT_Methodref_info constant. This constant contains the target method information, such as class name, method name, and method descriptor.

The isInstance method requires two elements on the operand stack: The first element is the type; the second element is the object. However, the isAssignableFrom method requires two type elements on the operand stack.

6.4. Summing Up

In summary, let's use a table to illustrate their differences:

Property instanceof Class.isInstance Class.isAssignableFrom
Kind keyword native method native method
Operands An object and a type A type and an object One type and another type
Null handling false false NullPointerException
Primitive types Not Supported Supported, but always false Yes
Class instance variable No Yes Yes
Bytecode instanceof invokevirtual invokevirtual
Most suitable when The object is given, type is known at compile time The object is given, the target type is not known at compile type No object is given, only types are known and only at runtime
Use cases Daily use, suitable for the majority of cases Complex and untypical cases such as implementing a library or a utility with the use of Reflection API

7. Conclusion

In this tutorial, we looked at the instanceof, Class.isInstance, and Class.isAssignableFrom methods and explored their usage and differences.

As always, the example code for this tutorial can be found over on GitHub.

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Get started with Spring 5 and Spring Boot 2, through the Learn Spring course:

>> CHECK OUT THE COURSE
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