Java synthetic method and bridge method confusion

Question

As stated in Effects of Type Erasure and Bridge Methods a bridge method implies that it is also a synthetic method. The question is, is it possible that invocation of isSynthetic method returns true but isBridge method returns false for a Method object, i.e., if a method is synthetic does it imply that it is also a bridge method?

It's not exactly the same but the source code for isSynthetic and isBridge methods looks like below:

static final int SYNTHETIC = 0x00001000;
public boolean isSynthetic() {
    return (getModifiers() & SYNTHETIC) != 0;
}

static final int BRIDGE = 0x00000040;
public boolean isBridge() {
    return (getModifiers() & BRIDGE) != 0;
}

Why isBridge method body is not like return isSynthetic();?

Answer 1

If you are simply looking for an example of such:

Function<String, Integer> func = s -> s.length();

Arrays.stream(DeleteMe.class.getDeclaredMethods())
     .peek(m -> System.out.println(m.getName() + " isSynth : " + m.isSynthetic() + " isBridge : " + m.isBridge()))
     .forEach(System.out::println);

There will be entries like:

lambda$0 isSynth : true isBridge : false
lambda$1 isSynth : true isBridge : false

Answer 2

Bridge is synthetic but synthetic is not necessarily bridge. Example:

public class Test {

    public Test clone() {
        return null;
    }

bytecode outline:

  // access flags 0x1041
  public synthetic bridge clone()Ljava/lang/Object; throws java/lang/CloneNotSupportedException 
   L0
...

Answer 3

is it possible that invocation of isSynthetic method returns true but isBridge method returns false for a Method object

Yes, it is possible.

For example, the method AbstractPipeline.lambda$spliterator$0() is synthetic but is it NOT bridge.

According to the JVM Spec:

• The ACC_SYNTHETIC flag indicates that this method was generated by a compiler and does not appear in source code
• The ACC_BRIDGE flag is used to indicate a bridge method generated by a compiler for the Java programming language.

So,

• A bridge method is 100% sure it is synthetic
• A synthetic method is not necessary bridge

More on bridge method

Based on What is Java Bridge Method (ACC_BRIDGE) the bridge method is generated by Java Compiler for type erasure purpose of Java Generics.