如何为集合实现toString()？

Question

我经常需要打印列表或地图，但我不喜欢它们默认的toString()方法的输出。例如，HashMap#toString()输出如下所示：

{key-1=value-1, key-2=value-2, key-3=value-3}

但我希望输出是这样的：

key-1 = value-1, key-2 = value-2, key-3 = value-3

我创建了一个扩展HashMap的自定义类，并编写了自己的toString()方法，这导致了我真正的问题：

public final class CustomMap<K, V> extends HashMap<K, V> {

    private static final long serialVersionUID = 1493227382148892732L;

    @Override
    public String toString() {
        final StringBuilder builder = new StringBuilder();

        final Iterator<Map.Entry<K, V>> entryIterator = entrySet().iterator();

        if (entryIterator.hasNext()) {
            final Map.Entry<K, V> entry = entryIterator.next();

            builder.append(entry.getKey());
            builder.append(" = ");
            builder.append(entry.getValue());
        }

        while (entryIterator.hasNext()) {
            final Map.Entry<K, V> entry = entryIterator.next();

            builder.append(", ");
            builder.append(entry.getKey());
            builder.append(" = ");
            builder.append(entry.getValue());
        }

        return builder.toString();
    }

}

正如您所看到的，我有重复的代码，因为,部分必须省略一次。那么，我如何在没有重复代码的情况下将其重写为一种快速有效的方法呢？

Answer 1

我刚刚检查了AbstractCollection的来源，就这样做了：

public final class CustomMap<K, V> extends HashMap<K, V> {

    /**
     * Serial version UID.
     */
    private static final long serialVersionUID = 1493227382148892732L;

    @Override
    public String toString() {
        final Iterator<Map.Entry<K, V>> entryIterator = entrySet().iterator();

        if (!entryIterator.hasNext()) {
            return "";
        }

        final StringBuilder builder = new StringBuilder();

        for (;;) {
            final Map.Entry<K, V> entry = entryIterator.next();

            builder.append(entry.getKey());
            builder.append(" = ");
            builder.append(entry.getValue());

            if (!entryIterator.hasNext()) {
                return builder.toString();
            }

            builder.append(", ");
        }
    }

}

这看起来非常聪明，而且循环中只有一个检查。

Answer 2

它可以很容易地用布尔值来解决：

    final Iterator<Map.Entry<K, V>> entryIterator = entrySet().iterator();
    boolean first = true;
    while (entryIterator.hasNext()) {
        final Map.Entry<K, V> entry = entryIterator.next();
        if (!first)
            builder.append(", ");
        else
            first=false;
        builder.append(entry.getKey());
        builder.append(" = ");
        builder.append(entry.getValue());
    }

另一种选择是使用Java 8 StringJoiner：

    StringJoiner sj = new StringJoiner(",");
    while (entryIterator.hasNext()) {
        final Map.Entry<K, V> entry = entryIterator.next();
        sj.add(entry.getKey() + " = " + entry.getValue());
    }
    return sj.toString();

Answer 3

不要扩展HashMap，而是在HashMap.toString()上使用String.subString()方法

String s=yourMap.toString();//eg-"{key-1=value-1, key-2=value-2, key-3=value-3}"
System.out.println(s.substring(1,s.length()-1)); 

输出： -key-1=value-1，key-2=value-2，key-3=value-3

或

创建删除最后逗号的方法。

public StringBuilder removeComma(StringBuilder sbf) {
                if (sbf.charAt(sbf.length() - 1) == ',') {
                    sbf.deleteCharAt(sbf.length() - 1);
                }
                return sbf;
            }


public String toString() {
        final StringBuilder builder = new StringBuilder();
        final Iterator<Map.Entry<K, V>> entryIterator = entrySet().iterator();
        while (entryIterator.hasNext()) {
            final Map.Entry<K, V> entry = entryIterator.next();
            builder.append(entry.getKey()).append(" = ").append(entry.getValue()).append(",");
        }
        return removeComma(builder).toString();
      }

使用变量另一个选项-

 public String toString() {
                String comma="";//initaly empty
                final StringBuilder builder = new StringBuilder();
                final Iterator<Map.Entry<K, V>> entryIterator = entrySet().iterator();
                while (entryIterator.hasNext()) {
                    final Map.Entry<K, V> entry = entryIterator.next();
                    builder.append(comma).append(entry.getKey()).append(" = ").append(entry.getValue());
                    comma=",";//set value of comma
                }
                return removeComma(builder).toString();
              }