如果未首先声明公共(public)类，则 Java 11 不会运行 main 方法

Question

我创建了以下 Chimpazee.java包含以下内容的文件:

class Primate {
    public Primate() {
        System.out.print("Primate-");
    }
}

class Ape extends Primate {
    public Ape(int fur) {
        System.out.print("Ape1-");
    }
    public Ape() {
        System.out.print("Ape2-");
    }
}

public class Chimpazee extends Ape {
    public Chimpazee() {
        super(2);
        System.out.print("Chimpazee-");
    }
    public static void main(String[] args) {
        new Chimpazee();
    }
}

但是，当我尝试使用 Java 11 在 Windows 10 + PowerShell 上执行此文件时...

PS C:\projects\ocp-java-se\java-se-11\chapter8> java -version
java version "11.0.11" 2021-04-20 LTS
Java(TM) SE Runtime Environment 18.9 (build 11.0.11+9-LTS-194)
Java HotSpot(TM) 64-Bit Server VM 18.9 (build 11.0.11+9-LTS-194, mixed mode)

我使用以下命令收到运行时错误

PS C:\projects\ocp-java-se\java-se-11\chapter8> java .\Chimpazee.java
error: can't find main(String[]) method in class: Primate

如果我将 Chimpazee 类移动到文件的开头，它工作正常。是不是假设 Java 编译这个与类的顺序无关？

Answer 1

JEP 330提到它:

The class to be executed is the first top-level class found in thesource file. It must contain a declaration of the standard publicstatic void main(String[]) method.

即您需要制作 public class Chimpanzee作为顶级类如下:

public class Chimpanzee extends Ape {
    public Chimpanzee() {
        super(2);
        System.out.print("Chimpanzee-");
    }
    public static void main(String[] args) {
        new Chimpanzee();
    }
}

class Primate {
    public Primate() {
        System.out.print("Primate-");
    }
}

class Ape extends Primate {
    public Ape(int fur) {
        System.out.print("Ape1-");
    }
    public Ape() {
        System.out.print("Ape2-");
    }
}

然后，运行它

java Chimpanzee.java

输出:

Primate-Ape1-Chimpanzee-