java - lambda 中的方法是否仅在执行 lambda 时才被评估？

Question

我们能否确定在 java lambda 中调用的方法仅在 lambda 本身执行时才被调用，而不是提前被调用？

如果你看看我的代码:

StringBuilder myStringBuilder = new StringBuilder("my result");
Supplier<String> mySupplier = () -> "result: " + myStringBuilder.toString();
myStringBuilder.append(", after appending.");
System.out.println(mySupplied.get());

我们能否 100% 确定结果将永远是:

result: my result, after appending

永远不要只是

result: my result

Answer 1

考虑以下快照:

Runnable runnable = () -> expensiveMethod();  // Runnable not called
firstMethodCall();                            // Runnable not called
secondMethodCall();                           // Runnable not called
runnable.run();                               // HERE IT COMES, Runnable IS called

lambda 表达式只不过是使用一种方法实现匿名类(这确保了 @FunctionalInterface 注释)。

Runnable runnable = new Runnable() {
     void run() {
         expensiveMethod();
     }
};

// runnable's method is not executed since the method run is not called
// the runnable.run() invokes the expensiveMethod()

Java 8 规范 15.27. Lambda Expressions解释当调用函数接口(interface)的适当方法时调用表达式:

Evaluation of a lambda expression produces an instance of a functional interface (§9.8). Lambda expression evaluation does not cause the execution of the expression's body; instead, this may occur at a later time when an appropriate method of the functional interface is invoked.