如何理解这个 Java 语法，方法名称前括号中的 class 名称？

Question

I'm seeing this syntax and can't understand what it means. It doesn't look like a regular object instantiation. The 'new' keyword isn't there and the class name is put in parenthesis... then there's a method call at the end.

MyClass myObj = (MyClass) someMethod(arg);

Answer 1

In Java,

someMethod(arg)

is a call of a method named someMethod with a single parameter arg .

All methods (except void ones) return a value. The type of the return value is part of method definition. In your case, we don't know what the someMethod returns (because you didn't provide its definition), but we can assume it is not void .

The expression

(MyClass) obj

is a type cast , which tells the compiler to treat obj as if it was of type MyClass . During compilation, compiler checks that the definition of obj is compatible with MyClass , and if it is, it allows you to treat it like one. During run time, Java virtual machine checks that the actual object obj is actually instance of class MyClass . If it is not, it will throw a ClassCastException .

Typically, type casts are used to "upgrade" how the compiler sees an object into something more specific. For example, if a pet shop by definition sells Animal s, and you ordered a Hamster , then you can reasonably assume thet what you get is not just an Animal , but a Hamster (assuming Hamster extends Animal ).

Now, put this together and you get a type cast applied to a method return value.

Answer 2

There is a method somewhere that is returning an object based on the arg arugment. The developer has provided a cast, to coerce the object into the MyClass type.

This is common in a Singleton pattern (where the same object instance is returned if the arg argument is the same), or in a static constructor. Usually the method is doing a new MySubclass(... ) somewhere and returning that.