java.lang.NullPointerException is thrown using a method-reference but not a lambda expression

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This behaviour relies on a subtle difference between the evaluation process of method-references and lambda expressions.

From the JLS Run-Time Evaluation of Method References:

First, if the method reference expression begins with an ExpressionName or a Primary, this subexpression is evaluated. If the subexpression evaluates to null, a NullPointerException is raised, and the method reference expression completes abruptly.

With the following code:

Thread t = new Thread(s::toLowerCase); // <-- s is null, NullPointerException thrown here
t.setUncaughtExceptionHandler((t1, e) -> System.out.println("Exception!"));

the expression s is evaluated to null and an exception is thrown exactly when that method-reference is evaluated. However, at that time, no exception handler was attached, since this code would be executed after.

This doesn’t happen in the case of a lambda expression, because the lambda will be evaluated without its body being executed. From Run-Time Evaluation of Lambda Expressions:

Evaluation of a lambda expression is distinct from execution of the lambda body.

Thread t = new Thread(() -> s.toLowerCase());
t.setUncaughtExceptionHandler((t1, e) -> System.out.println("Exception!"));

Even if s is null, the lambda expression will be correctly created. Then the exception handler will be attached, the thread will start, throwing an exception, that will be caught by the handler.

As a side-note, it seems Eclipse Mars.2 has a small bug regarding this: even with the method-reference, it invokes the exception handler. Eclipse isn’t throwing a NullPointerException at s::toLowerCase when it should, thus deferring the exception later on, when the exception handler was added.

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