相关疑难解决方法(0)

在这个简化的例子中,我有一个泛型类,以及一个返回Map而不管类型参数的方法.当我没有在包含类上指定类型时,为什么编译器会清除地图上的类型？

import java.util.Map;

public class MyClass<T>
{
    public Map<String, String> getMap()
    {   
        return null;
    }

    public void test()
    {   
        MyClass<Object> success = new MyClass<Object>();
        String s = success.getMap().get("");

        MyClass unchecked = new MyClass();
        Map<String, String> map = unchecked.getMap();  // Unchecked warning, why?
        String s2 = map.get("");

        MyClass fail = new MyClass();
        String s3 = fail.getMap().get("");  // Compiler error, why?
    }
}

我得到这个编译错误.

MyClass.java:20: incompatible types
found   : java.lang.Object
required: java.lang.String
                String s3 = fail.getMap().get("");  // Compiler error

当我编译下面的类(这里可用,添加main方法以在单个类中重现问题)时,我在IntelliJ或maven命令行中收到以下编译器警告:

java: /Users/luigi/var/src/owner/src/test/java/org/aeonbits/owner/multithread/ThreadBase.java uses unchecked or unsafe operations.
java: Recompile with -Xlint:unchecked for details.

然后我添加了-Xlint:unchecked to maven以查看详细信息,我得到了:

[WARNING] /Users/luigi/var/src/owner/src/test/java/org/aeonbits/owner/multithread/ThreadBase.java:[74,45] unchecked conversion
  required: java.util.List<java.lang.Throwable>
  found:    java.util.List

类源代码如下(已添加main以在单个类中重现问题):

package org.aeonbits.owner.multithread;

import org.aeonbits.owner.Config;
import org.aeonbits.owner.UtilTest.MyCloneable;

import java.util.ArrayList;
import java.util.List;

import static org.aeonbits.owner.UtilTest.debug;

abstract class ThreadBase<T extends Config> extends Thread implements MyCloneable {
    private static long counter = 0;
    final long uniqueThreadId = ++counter;
    final T cfg;
    final Object lock;
    final int loops;
    final List<Throwable> errors = new ArrayList<Throwable>();

    ThreadBase(T cfg, Object …

今天我遇到了javac关于泛型类型推理的奇怪行为.这是用于说明这种奇怪行为的示例类:

import java.util.Map;
import java.util.Collections;
import java.util.HashMap;
public class Test {

    protected <T> T strange(T t, Map<String, String> map) {
        return t;
    }

    protected void ok(Map<String, String> map) {}
    protected <T> T test(T t) {
        T res = strange(t , new HashMap<String, String>());
        //Doesn't work
        //res = strange(t, new <String, String>HashMap());
        ok(new <String, String>HashMap());
        res = strange(t, Collections.<String, String>emptyMap());
        //Doesn't work
        //res = strange(t, Collections.EMPTY_MAP);
        res = strange(t, (Map<String, String>) Collections.EMPTY_MAP);
        ok(Collections.EMPTY_MAP);
        return res;
    }
}

注意//Doesn't …