从类对象实例化类

Question

In java, can I use a class object to dynamically instantiate classes of that type?

ie I want some function like this.

Object foo(Class type) {
    // return new object of type 'type'
}

Answer 1

In Java 9 and afterward, if there's a declared zero-parameter ("nullary") constructor, you'd use Class.getDeclaredConstructor() to get it, then call newInstance() on it:

Object foo(Class type) throws InstantiationException, IllegalAccessException, InvocationTargetException {
    return type.getDeclaredConstructor().newInstance();
}

Prior to Java 9, you would have used Class.newInstance :

Object foo(Class type) throws InstantiationException, IllegalAccessException {
    return type.newInstance();
}

...but it was deprecated as of Java 9 because it threw any exception thrown by the constructor, even checked exceptions, but didn't (of course) declare those checked exceptions, effectively bypassing compile-time checked exception handling. Constructor.newInstance wraps exceptions from the constructor in InvocationTargetException instead.

Both of the above assume there's a zero-parameter constructor. A more robust route is to go through Class.getDeclaredConstructors or Class.getConstructors , which takes you into using the Reflection stuff in the java.lang.reflect package, to find a constructor with the parameter types matching the arguments you intend to give it.

Answer 2

Use:

type.newInstance()

For creating an instance using the empty costructor, or use the method type.getConstructor(..) to get the relevant constructor and then invoke it.

Answer 3

Yes, it is called Reflection. you can use the Class newInstance() method for this.

Answer 4

使用 newInstance() 方法。