ArrayList remove

1.removeIf

public boolean removeIf(Predicate<? super E> filter) {  
    int removeCount = 0;
    // 位图
    final BitSet removeSet = new BitSet(size);
    final int expectedModCount = modCount;
    final int size = this.size;
    for (int i=0; modCount == expectedModCount && i < size; i++) {  
        final E element = (E) elementData[i];
        // 条件满足
        if (filter.test(element)) {
            // 设置为true
            removeSet.set(i);
            removeCount++;
        }
    }
    if (modCount != expectedModCount) {
        throw new ConcurrentModificationException();
    }
    final boolean anyToRemove = removeCount > 0;
    if (anyToRemove) {
        final int newSize = size - removeCount;
        for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {
            // 返回>=i的值为false的最近的下标
            i = removeSet.nextClearBit(i);
            elementData[j] = elementData[i];
        }
        for (int k=newSize; k < size; k++) {
            elementData[k] = null;
        }
        this.size = newSize;
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
        modCount++;
    }
    return anyToRemove;
}

2.removeAll

public boolean removeAll(Collection<?> c) {
    return batchRemove(c, false);
}
private boolean batchRemove(Collection<?> c, boolean complement) {
    final Object[] elementData = this.elementData;
    int r = 0, w = 0;
    boolean modified = false;
    try {
        for (; r < size; r++)
            if (c.contains(elementData[r]) == complement)
                elementData[w++] = elementData[r];
    } finally {
        if (w != size) {
            for (int i = w; i < size; i++)
                elementData[i] = null;
            modCount += size - w;
            size = w;
            modified = true;
        }
    }
    return modified;
}

3.iterator

private class Itr implements Iterator<E> {
    int cursor;
    int lastRet = -1;
    int expectedModCount = modCount;
    Itr() {}
    public boolean hasNext() {
        return cursor != size;
    }
    public E next() {
        checkForComodification();
        int i = cursor;
        if (i >= size)
            throw new NoSuchElementException();
        Object[] elementData = ArrayList.this.elementData;
        if (i >= elementData.length)
            throw new ConcurrentModificationException();
        cursor = i + 1;
        return (E) elementData[lastRet = i];
    }
    public void remove() {
        if (lastRet < 0)
            throw new IllegalStateException();
        checkForComodification();
        try {
            ArrayList.this.remove(lastRet);
            cursor = lastRet;
            lastRet = -1;
            expectedModCount = modCount;
        } catch (IndexOutOfBoundsException ex) {
            throw new ConcurrentModificationException();
        }
    }
    final void checkForComodification() {
        if (modCount != expectedModCount)
            throw new ConcurrentModificationException();
    }
}

4.从后往前遍历

List<Long> list = new ArrayList<>(Arrays.asList(1L,2L,2L,4L,5L));
for (int i = list.size() - 1; i >= 0; i--) {
    if (list.get(i) == 2) {
        list.remove(i);
    }
}

5.i—

List<Long> list = new ArrayList<>(Arrays.asList(1L, 2L, 2L, 4L, 5L));
for (int i = 0; i < list.size(); i++) {
    if (list.get(i) == 2) {
        list.remove(i);
        i--;
    }
}