Java中Group类嵌套结构toString方法缩进异常问题

我在可容纳Figure及其他Group的Group类中实现了toString方法，期望输出带有层级缩进的嵌套Group文本结构，但实际调用时所有子元素仅显示一个制表符缩进，无法呈现预期的层级效果。尝试将制表符替换为双制表符后，问题仍未解决。

预期输出 #

Group consists of:
    Group consists of:
        Rectangle with center at (20.0, 10.0), and width: 15.0, height: 15.0, angle: 45.0
        Rectangle with center at (0.0, 0.0), and width: 7.0, height: 19.0, angle: 90.0
    Group consists of:
        Circle with center at (100.0, 100.0) and radius: 1.0
        Circle with center at (5.0, 0.0) and radius: 9.0

    Rectangle with center at (10.0, 10.0), and width: 10.0, height: 40.0, angle: 0.0
    Circle with center at (0.0, 0.0) and radius: 55.0

实际错误输出 #

Group consists of:
    Group consists of:
    Rectangle with center at (10.0, 10.0), and width: 10.0, height: 15.0, angle: 0.0
    Rectangle with center at (20.0, 10.0), and width: 15.0, height: 15.0, angle: 45.0
    Rectangle with center at (0.0, 0.0), and width: 7.0, height: 19.0, angle: 90.0
    Group consists of:
    Circle with center at (0.0, 0.0) and radius: 10.0
    Circle with center at (100.0, 100.0) and radius: 1.0
    Circle with center at (5.0, 0.0) and radius: 9.0

初始实现代码 #

public String toString() {
        StringBuilder str = new StringBuilder("Group consists of:\n");
        for (BaseFigure figure : this.group) {
            str.append("\t").append(figure.toString()).append("\n");
        }
        return str.toString();
    }

尝试修改后的代码 #

public String toString() {
        String str = "Group consists of:\n";
        for (BaseFigure figure : this.group) {
            if (figure instanceof Group) {
                String newStr = figure.toString();
                str += newStr.replace("\t", "\t\t");
            } else {
                str += figure.toString();
            }
        }
        return str;
    }

解决方案 #

核心问题是没有将当前层级的缩进传递给子Group，导致子Group的所有内容仅使用自身的基础缩进，无法叠加父级的缩进层级。正确的做法是通过重载toString方法，增加缩进参数来传递层级信息，递归调用时加深缩进：

修正后的代码 #

// 对外暴露的无参方法，初始化递归的缩进为空
@Override
public String toString() {
    return toString("");
}

// 私有递归方法，接收当前层级的缩进参数
private String toString(String indent) {
    StringBuilder sb = new StringBuilder(indent + "Group consists of:\n");
    // 子元素的缩进是当前缩进追加一个制表符
    String childIndent = indent + "\t";
    
    for (BaseFigure figure : this.group) {
        if (figure instanceof Group) {
            // 递归调用子Group的带参toString，传递子层级缩进
            sb.append(((Group) figure).toString(childIndent));
        } else {
            // 普通Figure直接使用子层级缩进拼接内容
            sb.append(childIndent).append(figure.toString()).append("\n");
        }
    }
    return sb.toString();
}

说明 #

1. 用带缩进参数的私有方法处理递归逻辑，每次进入子Group时，将当前缩进加上一个制表符传递下去，确保子Group的所有内容基于父级缩进再加深一层。
2. 无参的toString作为对外接口，初始缩进为空字符串，保证最外层Group没有前置缩进。
3. 普通Figure元素直接使用当前层级的子缩进，确保与同一层级的Group内容对齐。

这样就能生成符合预期的层级缩进结构了。

内容的提问来源于stack exchange，提问作者Fischer Benkovscki