如何在 Java 中編寫通用的 foreach 循環？

Question

到目前為止，我有以下代碼：

/*
 * This method adds only the items that don’t already exist in the
 * ArrayCollection. If items were added return true, otherwise return false.
 */
public boolean addAll(Collection<? extends E> toBeAdded) {

    // Create a flag to see if any items were added
    boolean stuffAdded = false;


    // Use a for-each loop to go through all of the items in toBeAdded
    for (something : c) {

        // If c is already in the ArrayCollection, continue
        if (this.contains(c)) { continue; }     

            // If c isn’t already in the ArrayCollection, add it
            this.add(c)
            stuffAdded = true;
        }

        return stuffAdded;
    }
}

我的問題是：我應該用什么替換某些東西（和 c）來完成這項工作？

Answer 1

這樣的事情應該做：

// Use a for-each loop to go through all of the items in toBeAdded
for (E c : toBeAdded) {

    // If c is already in the ArrayCollection, continue
    if (this.contains(c)) {
        continue;
    }

    // If c isn’t already in the ArrayCollection, add it
    this.add(c);

    stuffAdded = true;
}

---

一般形式為：

for (TypeOfElements iteratorVariable : collectionToBeIteratedOver) `

Answer 2

E 是集合，c 是循環內的變量 for(E c: toBeAdded )...

Answer 3

在 Java 中寫一個 foreach 非常簡單。

for(ObjectType name : iteratable/array){ name.doSomething() }

您可以使用可迭代或數組執行 foreach。 請注意，如果您不對迭代器（Iterator）進行類型檢查，那么您需要使用 Object 作為 ObjectType。 否則使用 E 是什么。 例如

ArrayList<MyObject> al = getObjects();
for(MyObject obj : al){
  System.out.println(obj.toString());
}

對於您的情況：

   for(E c : toBeAdded){
        // If c is already in the ArrayCollection, continue
        if( this.contains(c) ){ continue;}     

        // If c isn’t already in the ArrayCollection, add it
        this.add(c)
        stuffAdded = true;
    }

Answer 4

您可以用 2 種方式編寫 foreach 循環，它們是等效的。

List<Integer> ints = Arrays.asList(1,2,3);
int s = 0;
for (int n : ints) { s += n; }

---

for (Iterator<Integer> it = ints. iterator(); it.hasNext(); ) {
    int n = it.next();
    s += n;
}

Answer 5

public boolean addAll(Collection<? extends E> toBeAdded) {
    boolean stuffAdded = false;
    for(E c : toBeAdded){
        if(!this.contains(c)){
             this.add(c)
             stuffAdded = true;
        }
    }
    return stuffAdded;
}

另請查看Collections.addAll