为什么我在使用 Runtime 时不需要导入 java.lang？

Question

In the piece of code below, Why do I need to import java.awt.Robot but I can use Runtime without importing java.lang.Runtime ?

package classes;

import java.awt.Robot;

public class RuntimeExecution {
    public static void main(String[] args) {
        try {
            Runtime.getRuntime().exec("notepad");
            Robot robot = new Robot();
            robot.keyPress(65);
        } catch (Exception e) {
            System.out.println(e.getMessage());
        }
    }
}

Answer 1

Because

Every compilation unit implicitly imports every public type name declared in the predefined package java.lang , as if the declaration import java.lang.*; appeared at the beginning of each compilation unit immediately after any package statement. As a result, the names of all those types are available as simple names in every compilation unit.

where a compilation unit is basically your .java source file.

Answer 2

For convenience, the Java compiler automatically imports two entire packages for each source file:

1. The java.lang package and
2. The current package (the package for the current file).

Please ref. oracle.com for more info (http://docs.oracle.com/javase/tutorial/java/package/usepkgs.html)