Java .contains 方法

Question

这个问题已经有答案了:

Java ArrayList Contain always return false although it contain the same value (4 个回答)

已关闭 4 年前。

我创建了一个充满“状态”的数组列表，但添加状态后在列表中找不到状态

public class State {
    int a;
    int b;
    int c;

    public State(int a,int b,int c) {
        super();
        this.a = a;
        this.b = b;
        this.c = c;
    }
}

然后在主类中

public class Main {
    static ArrayList<State> nodes = new ArrayList<State>();

    public static void main(String[] args) {
      State randomState = new State(12,0,0);
      nodes.add(randomState);
      System.out.println(nodes.contains(new State(12,0,0)));
    }      
}

这将返回 false，但如果我这样做

System.out.println(nodes.contains(randomState));

将返回 true。任何帮助表示赞赏

Answer 1

List.contains()依赖于对象的 equals() 方法:

More formally, returns true if and only if this list contains at least one element e such that (o==null ? e==null : o.equals(e)).

在 State 类中覆盖它和 hashCode()，例如:

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (!(o instanceof State)) return false;
    State state = (State) o;
    return a == state.a &&
            b == state.b &&
            c == state.c;
}

@Override
public int hashCode() {
    return Objects.hash(a, b, c);
}

或者不要使用此方法并自行执行搜索。
例如:

public boolean isAnyMatch(List<State> states, State other){   
  return states.stream()
               .anyMatch(s -> s.getA() == other.getA() && 
                         s.getB() == other.getB()  && 
                         s.getC() == other.getC() )
}


System.out.println(isAnyMatch(nodes, new State(12,0,0));