JavaコレクションクラスのtoString()をスレッドローカル変数で補強する ref: http://qiita.com/yuba/items/35fada18850578edea73

AbstractCollection.java

public String toString() {
    Iterator<E> it = iterator();
    if (! it.hasNext())
        return "[]";

    StringBuilder sb = new StringBuilder();
    sb.append('[');
    for (;;) {
        E e = it.next();
        sb.append(e == this ? "(this Collection)" : e);
        if (! it.hasNext())
            return sb.append(']').toString();
        sb.append(',').append(' ');
    }
}

file1.java

private static ThreadLocal<Set<Object>> stackedObjectsContainer = new ThreadLocal<>();

@Override
public String toString() {
    Iterator<E> it = iterator();
    if (! it.hasNext())
        return "[]";

    Set<Object> stackedObjects = stackedObjectsContainer.get();
    if (stackedObjects == null) {
        stackedObjects = new HashSet<Object>();
        stackedObjectsContainer.set(stackedObjects);
    }
    stackedObjects.add(this);
    try {
        StringBuilder sb = new StringBuilder();
        sb.append('[');
        for (; ; ) {
            E e = it.next();
            sb.append(stackedObjects.contains(e) ? "(recursion)" : e);
            if (!it.hasNext())
                return sb.append(']').toString();
            sb.append(',').append(' ');
        }
    } finally {
        stackedObjects.remove(this);
    }
}