Java类成员变量赋值报错：为何无法直接设置left与right的关联？

错误原因 #

Java类体中仅允许定义成员变量、方法、初始化块、内部类或枚举等元素，不能直接在类体中执行赋值语句（如left.next = right;）。编译器会将类体中不属于合法成员定义的代码当作成员声明处理，因此找不到预期的标识符，抛出<identifier> expected错误。

修复方案 #

方案1：使用构造方法初始化 #

将节点关联逻辑放到类的构造方法中，创建Solution实例时自动执行初始化：

import java.util.HashMap;
import java.util.Map;

public class Solution {
    Map<Integer, Node> map = new HashMap<>();
    public Node left = new Node(Integer.MIN_VALUE, null, null);
    public Node right = new Node(Integer.MAX_VALUE, null, null);

    // 构造方法中完成节点关联
    public Solution() {
        left.next = right;
        right.prev = left;
    }

    public static void main(String[] args) {
        System.out.println("");
    }
}

class Node {
    Node prev;
    Node next;
    int val;
    public Node(int val, Node prev, Node next) {
        this.val = val;
        this.prev = prev;
        this.next = next;
    }
}

方案2：使用实例初始化块 #

用大括号包裹初始化逻辑，作为实例初始化块，每次创建Solution实例时执行：

import java.util.HashMap;
import java.util.Map;

public class Solution {
    Map<Integer, Node> map = new HashMap<>();
    public Node left = new Node(Integer.MIN_VALUE, null, null);
    public Node right = new Node(Integer.MAX_VALUE, null, null);

    // 实例初始化块
    {
        left.next = right;
        right.prev = left;
    }

    public static void main(String[] args) {
        System.out.println("");
    }
}

class Node {
    Node prev;
    Node next;
    int val;
    public Node(int val, Node prev, Node next) {
        this.val = val;
        this.prev = prev;
        this.next = next;
    }
}

原问题代码与错误信息 #

原代码 #

public class Solution {
    Map<Integer, Node> map = new HashMap<>();
    public Node left = new Node(Integer.MIN_VALUE, null, null);
    public Node right = new Node(Integer.MAX_VALUE, null, null);
    left.next = right;
    right.prev = left;

    public static void main(String[] args) {
        System.out.println("");
    }
}

class Node {
    Node prev;
    Node next;
    int val;
    public Node(int val, Node prev, Node next) {
        this.val = val;
        this.prev = prev;
        this.next = next;
    }
}

编译错误 #

Solution.java:14: error: <identifier> expected
    left.next = right;
             ^
Solution.java:15: error: <identifier> expected
    right.prev = left;
              ^

内容的提问来源于stack exchange，提问作者tomday