When does JVM decide to reuse old lambda?

Question

Consider the following code fragment:

public static Object o = new Object();

public static Callable x1() {
    Object x = o;
    return () -> x;
}

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Answer 1

You can't be sure about the identity of the object returned for a lambda expression. It can be a new instance, or a pre-existing instance.

This is specified in JLS §15.27.4:

At run time, evaluation of a lambda expression is similar to evaluation of a class instance creation expression, insofar as normal completion produces a reference to an object. Evaluation of a lambda expression is distinct from execution of the lambda body.

Either a new instance of a class with the properties below is allocated and initialized, or an existing instance of a class with the properties below is referenced. If a new instance is to be created, but there is insufficient space to allocate the object, evaluation of the lambda expression completes abruptly by throwing an OutOfMemoryError.