private static final int DEFAULT_CAPACITY = 10;//初始容量为10
private static final Object[] EMPTY_ELEMENTDATA = {};
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
transient Object[] elementData;
private int size;
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
//ArrayList扩容函数方法
private void ensureExplicitCapacity(int minCapacity) {
    modCount++;

    // overflow-conscious code
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}
private void grow(int minCapacity) {
    // 计算当前ArrayList大小
    int oldCapacity = elementData.length;
    //这里我们可以看出，ArrayList每次扩容是增加50%，oldCapacity >> 1是指往左移一位，也就是除以2
    int newCapacity = oldCapacity + (oldCapacity >> 1);
    if (newCapacity - minCapacity < 0)
        newCapacity = minCapacity;
    if (newCapacity - MAX_ARRAY_SIZE > 0)
        newCapacity = hugeCapacity(minCapacity);
    // minCapacity is usually close to size, so this is a win:
    elementData = Arrays.copyOf(elementData, newCapacity);
}
//根据下标index获取元素值
public E get(int index) {
    rangeCheck(index);//检查小标是否越界
    return elementData(index);
}
//将index位置的值设为element，并返回原来的值
public E set(int index, E element) {
    rangeCheck(index);

    E oldValue = elementData(index);
    elementData[index] = element;
    return oldValue;
}
//向数组中末尾增加一个元素
public boolean add(E e) {
    ensureCapacityInternal(size + 1);  // Increments modCount!!
    elementData[size++] = e;
    return true;
}
//向指定位置index处增加element
public void add(int index, E element) {
    rangeCheckForAdd(index);

    ensureCapacityInternal(size + 1);  // Increments modCount!!
    //将index以及index之后的数据复制到index+1的位置往后，即从index开始向后挪了一位
    System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
    elementData[index] = element;
    size++;
}
//根据指定的index，删除元素
public E remove(int index) {
    rangeCheck(index);

    modCount++;
    E oldValue = elementData(index);

    int numMoved = size - index - 1;
    if (numMoved > 0)
        System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
    elementData[--size] = null; // clear to let GC do its work

    return oldValue;
}

package java.util;public class LinkedList<E> extends AbstractSequentialList<E>
 implements List<E>, Deque<E>, Cloneable, java.io.Serializable {
    transient int size = 0;
    transient Node<E> first;
    transient Node<E> last;
}

String a = "abc";
a = a + "d";
System.out.println(a);
StringBuffer buffer = new StringBuffer();
buffer.append("a");
System.out.println(buffer);
StringBuilder builder = new StringBuilder();
builder.append("b");
System.out.println(builder);

Map<Integer,String> map = new HashMap<>();

class MyClass{
    //必要的参数
    private final int one;
    private final int two;
    //可选的参数
    private final int three;
    private final int four;
    public MyClass1(int one,int two){
        this(one,two,0);
    }
    public MyClass(int one,int two,int three){
        this(one,two,three,0);
    }
    public MyClass(int one,int two,int three,int four){
        this.one=one;
        this.two=two;
        this.three=three;
        this.four=four;
    }
    @Override
    public String toString(){
        return " one="+this.one+" two="+this.two+" three="+this.three+" four="+this.four;
    }
}

String str = "1,2,3,4,10,11,9,66,222,12";
List<Integer> list = Stream.of(str.split(","))
                .map(Integer::valueOf)
                .filter(x-> !Objects.equals(x,3))
                .sorted(Comparator.reverseOrder())
                .limit(4)
                .collect(Collectors.toList());

@Data
public class ListDTO{
    public Integer id;
    public String name;
    public ListDTO() {
    }
    public ListDTO(Integer id, String name) {
        this.id = id;
        this.name = name;
    }
}

List<Integer> idList = beans.stream().map(ListDTO::getId).collect(Collectors.toList());

LocalTime localTime = LocalTime.now();
LocalDate localDate = LocalDate.now();
LocalDateTime localDateTime = LocalDateTime.of(localDate, localTime);

LocalDateTime localDateTime = LocalDateTime.now();

DateTimeFormatter dateformatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");
LocalDate ld = LocalDate.parse("2017-08-01", dateformatter);