In Java Lambda’s why is getClass() called on a captured variable

Yes, calling getClass() has become a canonical “test for null” idiom, as getClass() is expected to be a cheap intrinsic operation and, I suppose, HotSpot might be capable of detecting this pattern and reduce the operation to an intrinsic null-check operation, if the result of getClass() is not used.

Another example is creating an inner class instance with an outer instance that is not this:

public class ImplicitNullChecks {
    class Inner {}
    void createInner(ImplicitNullChecks obj) {
        obj.new Inner();
    }

    void lambda(Object o) {
        Supplier<String> s=o::toString;
    }
}

compiles to

Compiled from "ImplicitNullChecks.java"
public class bytecodetests.ImplicitNullChecks {
  public bytecodetests.ImplicitNullChecks();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  void createInner(bytecodetests.ImplicitNullChecks);
    Code:
       0: new           #23                 // class bytecodetests/ImplicitNullChecks$Inner
       3: dup
       4: aload_1
       5: dup
       6: invokevirtual #24                 // Method java/lang/Object.getClass:()Ljava/lang/Class;
       9: pop
      10: invokespecial #25                 // Method bytecodetests/ImplicitNullChecks$Inner."<init>":(Lbytecodetests/ImplicitNullChecks;)V
      13: pop
      14: return

  void lambda(java.lang.Object);
    Code:
       0: aload_1
       1: dup
       2: invokevirtual #24                 // Method java/lang/Object.getClass:()Ljava/lang/Class;
       5: pop
       6: invokedynamic #26,  0             // InvokeDynamic #0:get:(Ljava/lang/Object;)Ljava/util/function/Supplier;
      11: astore_2
      12: return
}

See also JDK-8073550:

A few places in our class library use the weird trick of using object.getClass() to check for nullity.
While this make seem a smart move, it actually confuses people into believing this is an approved
practice of null checking.

With JDK 7, we have Objects.requireNonNull that provide the proper null checking, and declare the
intent properly.

It might be debatable whether this should apply to programming language intrinsic checks as well, as using Objects.requireNonNull for that purpose would create a dependency to a class outside the java.lang package not visible in the source code. And in this specific case, the trick is only visible to those who look at the byte code. But it has been decided to change the behavior with Java 9.

This is how jdk1.9.0b160 compiles the same test class:

Compiled from "ImplicitNullChecks.java"
public class bytecodetests.ImplicitNullChecks {
  public bytecodetests.ImplicitNullChecks();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  void createInner(bytecodetests.ImplicitNullChecks);
    Code:
       0: new           #26                 // class bytecodetests/ImplicitNullChecks$Inner
       3: dup
       4: aload_1
       5: dup
       6: invokestatic  #27                 // Method java/util/Objects.requireNonNull:(Ljava/lang/Object;)Ljava/lang/Object;
       9: pop
      10: invokespecial #28                 // Method bytecodetests/ImplicitNullChecks$Inner."<init>":(Lbytecodetests/ImplicitNullChecks;)V
      13: pop
      14: return

  void lambda(java.lang.Object);
    Code:
       0: aload_1
       1: dup
       2: invokestatic  #27                 // Method java/util/Objects.requireNonNull:(Ljava/lang/Object;)Ljava/lang/Object;
       5: pop
       6: invokedynamic #29,  0             // InvokeDynamic #0:get:(Ljava/lang/Object;)Ljava/util/function/Supplier;
      11: astore_2
      12: return
}