Java虚拟机是否允许在返回类型上重载？

Question

我已经完成了这个演讲.

幻灯片编号:26引用

Java language does not allow overloading on return type

Java Virtual machine does allow overloading on return type

这些陈述是真的吗？如果两个语句都为真,那么如何使代码可编译以便jvm运行代码？

我有一个关于这个主题的SE问题:

Java - 为什么没有基于返回类型的方法重载？

提前致谢.

Answer 1

这些陈述完全正确.

请记住,Java是两件事 - 一个是语言,两个是虚拟机.虽然限制语言不允许基于类型的方法重载使Java成为一种更简单的语言,但JVM仍然可以允许这样做以使其更强大.

作为一种语言,Java有一个编译器,它强制执行使Java成为一种更简单,更容易的编程语言的规则.为此,它限制了您可以执行的操作,但仅限于Java语言本身.在JVM上运行类似Scala或Ruby的东西需要不同的规则和功能,并且在这个级别上,JVM允许灵活性使JVM在如此众多的平台和设备上获得如此巨大的成功是非常重要的.

在一种可以通过返回类型进行重载的语言中,它将非常容易出错,并且不支持该功能的决定是故意使Java成为一种不易出错的编程语言.编译器如何知道您打算调用哪个函数？

另一方面,JVM是一个低级别,高度优化的虚拟机,用于运行字节码,而不是Java.因此,以这种方式限制JVM是不明智的,因为它应该能够运行不是从Java生成的字节码.

另一个例子是多重继承,这在Java中是不可用的,但是没有什么能阻止你编写支持多继承并将其编译为字节码的语言.它会使您的语言更难以使用并且可能更容易出错,但如果您需要该功能,JVM将不会阻止您.

Answer 2

除了由埃瓦尔德答案,这里是一个小的演示显示,它实际上是可能的过载基于返回类型有.可以有一个具有两个具有相同名称和参数的方法的类,它们只在返回类型上有所不同.

_{(事实上​​,这是一个纯粹的Java程序,模糊了语言和JVM之间的界限,并且回答了"Java"是否允许这种超载更加困难的问题,但我认为Ewald已经很好地解释了这一点 - 并且当然,我为这个演示"作弊"...... :)}

该程序使用Apache ByteCode工程库(BCEL)在运行时生成并加载这样的类.然后,它创建此类的实例,并列出并调用所有(声明的)方法.

package stackoverflow.returntypes;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;

import org.apache.bcel.Constants;
import org.apache.bcel.generic.ClassGen;
import org.apache.bcel.generic.ConstantPoolGen;
import org.apache.bcel.generic.InstructionFactory;
import org.apache.bcel.generic.InstructionList;
import org.apache.bcel.generic.MethodGen;
import org.apache.bcel.generic.ObjectType;
import org.apache.bcel.generic.PUSH;
import org.apache.bcel.generic.Type;

public class DifferentReturnTypesDemo
{
    public static void main(String[] args) throws Exception
    {
        ClassGenerator classGenerator = new ClassGenerator();
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        classGenerator.create(baos);

        ByteArrayClassLoader byteArrayClassLoader = new ByteArrayClassLoader(
            baos.toByteArray());
        Class<?> c = byteArrayClassLoader.loadClass(
            "stackoverflow.returntypes.Generated");
        byteArrayClassLoader.close();
        Object instance = c.newInstance();
        for (Method method : c.getDeclaredMethods())
        {
            System.out.println(method);
            method.invoke(instance, (Object[]) null);
        }
    }
}

class ByteArrayClassLoader extends URLClassLoader
{
    private final byte data[];

    ByteArrayClassLoader(byte data[])
    {
        super(new URL[0]);
        this.data = data;
    }

    @Override
    protected Class<?> findClass(final String name)
        throws ClassNotFoundException
    {
        return defineClass(name, data, 0, data.length);
    }
}

class ClassGenerator
{
    private InstructionFactory instructionFactory;
    private ConstantPoolGen constantPool_cp;
    private ClassGen classGen;

    public ClassGenerator()
    {
        classGen = new ClassGen("stackoverflow.returntypes.Generated",
            "java.lang.Object", "Generator.java", Constants.ACC_PUBLIC |
                Constants.ACC_SUPER, new String[] {});

        constantPool_cp = classGen.getConstantPool();
        instructionFactory = new InstructionFactory(classGen, constantPool_cp);
    }

    public void create(OutputStream out) throws IOException
    {
        createCreateConstructor();
        createMethodReturningInt();
        createMethodReturningFloat();
        classGen.getJavaClass().dump(out);
    }

    private void createCreateConstructor()
    {
        InstructionList instructionList = new InstructionList();
        MethodGen method = new MethodGen(Constants.ACC_PUBLIC, Type.VOID,
            Type.NO_ARGS, new String[0], "<init>",
            "stackoverflow.returntypes.Generated", instructionList,
            constantPool_cp);
        instructionList.append(InstructionFactory.createLoad(Type.OBJECT, 0));
        instructionList.append(instructionFactory.createInvoke(
            "java.lang.Object", "<init>", Type.VOID, Type.NO_ARGS,
            Constants.INVOKESPECIAL));
        instructionList.append(InstructionFactory.createReturn(Type.VOID));
        method.setMaxStack();
        method.setMaxLocals();
        classGen.addMethod(method.getMethod());
        instructionList.dispose();
    }

    private void createMethodReturningInt()
    {
        // Create a public, no-arguments method named "print" that
        // returns an int value
        InstructionList instructionList = new InstructionList();
        MethodGen method = new MethodGen(Constants.ACC_PUBLIC, Type.INT,
            Type.NO_ARGS, new String[0], "print",
            "stackoverflow.returntypes.Generated", instructionList,
            constantPool_cp);

        // Generate the "System.out.println" instructions
        instructionList.append(instructionFactory.createFieldAccess(
            "java.lang.System", "out", new ObjectType("java.io.PrintStream"),
            Constants.GETSTATIC));
        instructionList.append(new PUSH(constantPool_cp, "return int"));
        instructionList.append(instructionFactory.createInvoke(
            "java.io.PrintStream", "println", Type.VOID,
            new Type[] { Type.STRING }, Constants.INVOKEVIRTUAL));

        // Generate the return instruction
        instructionList.append(new PUSH(constantPool_cp, 123));
        instructionList.append(InstructionFactory.createReturn(Type.INT));

        method.setMaxStack();
        method.setMaxLocals();
        classGen.addMethod(method.getMethod());
        instructionList.dispose();
    }

    private void createMethodReturningFloat()
    {
        // Create a public, no-arguments method named "print" that
        // returns a float value
        InstructionList instructionList = new InstructionList();
        MethodGen method = new MethodGen(Constants.ACC_PUBLIC, Type.FLOAT,
            Type.NO_ARGS, new String[0], "print",
            "stackoverflow.returntypes.Generated", instructionList,
            constantPool_cp);

        // Generate the "System.out.println" instructions
        instructionList.append(instructionFactory.createFieldAccess(
            "java.lang.System", "out", new ObjectType("java.io.PrintStream"),
            Constants.GETSTATIC));
        instructionList.append(new PUSH(constantPool_cp, "return float"));
        instructionList.append(instructionFactory.createInvoke(
            "java.io.PrintStream", "println", Type.VOID,
            new Type[] { Type.STRING }, Constants.INVOKEVIRTUAL));

        // Generate the return instruction
        instructionList.append(new PUSH(constantPool_cp, 456.789f));
        instructionList.append(InstructionFactory.createReturn(Type.FLOAT));

        method.setMaxStack();
        method.setMaxLocals();
        classGen.addMethod(method.getMethod());
        instructionList.dispose();
    }
}

输出是

public int stackoverflow.returntypes.Generated.print()
return int
public float stackoverflow.returntypes.Generated.print()
return float

显示具有相同名称签名的方法出现两次,只在返回类型上有所不同 - 并且,使用反射,仍然可以调用这两种方法.