java - 通过 Java 中的另一个函数传递字典值时通过引用更新字典值

Question

我是 Java 新手，想知道是否可以在字典本身之外更新字典的值。我认为我通过在 ID2 中的“process”方法中创建一个新实例来失去与“nodesInincluded”集的连接。有没有什么巧妙的方法可以始终跟踪字典中值下面的对象。

抱歉，我最初粘贴了错误的代码:)

public class Dictionary {

    @AllArgsConstructor
    private class Info {
        private Double average;
        private Set nodesIncluded;
    }

    private void initialize(){
        Map<String, Info> testDict = new HashMap<String, Info>(){{
        put("ID001", new Info(1.0d, new HashSet<>(Arrays.asList("a", "b"))));
        put("ID002", null);
        }};

        Info takenOut01 = testDict.get("ID001");
        Info takenOut02 = testDict.get("ID002");

        process(takenOut01, "c");
        process(takenOut02, "c");

        System.out.print(testDict.get("ID001").nodesIncluded.contains("c")); // true
        System.out.print(testDict.get("ID002").nodesIncluded.contains("c")); // NullPointerException
        System.out.print(testDict.get("ID002").nodesIncluded.contains("default")); // NullPointerException
    }

    private void process (Info takenOut, String newValue ) {
        if (takenOut == null) {
            takenOut = new Info(0.0d, new HashSet<>(Arrays.asList("default")));
        }
        takenOut.nodesIncluded.add(newValue);
    }

    public static void main(String[] args) {
        Dictionary test = new Dictionary();
        test.initialize();
    }
}

Answer 1

Java 按值传递，但是当涉及到对象时，就有点困惑了。当对象通过方法传递时，它的引用值也会被传递。这意味着在方法范围内对对象执行的操作会影响其范围之外的对象。

public static class Bar {
    private int num = 0;
    public int getNum() { return num; }
    public void setNum(int num) { this.num = num; }
}

public static void setBar5(Bar bar) {
    bar.setNum(5);
    System.out.println(bar.getNum()); // outputs 5
}

public static void main(String[] args) {
    Bar bar = new Bar();
    setBar5(bar);

    System.out.println(bar.getNum());
}

简而言之，如果您持有对 Collection 内的对象的对象引用，则对其进行的任何操作都会看起来像您正在修改它。

Bar bar = new Bar();
List<Bar> bars = new ArrayList<>();
bars.add(bar);
bar.setNum(5);

for(Bar b : bars) {
    System.out.println(b.getNum());
}
// outputs 5

如果从集合中获取对象并修改它，它也会影响保存该对象的任何变量。

Bar bar2 = bar;

// modifying the element in the collection
bars.get(0).setNum(4);

// Even when we modify bar, bar2 gets affected.
System.out.println(bar2.getNum()); // outputs 4