对于想了解java.util.ArrayList的读者，本文将是一篇不可错过的文章，我们将详细介绍java.util.arraylistcannotbe，并且为您提供关于'java.lang.NoCl

对于想了解java.util.ArrayList的读者，本文将是一篇不可错过的文章，我们将详细介绍java.util.arraylist cannot be，并且为您提供关于'java.lang.NoClassDefFoundError 由 java.lang.ClassNotFoundException 引起的 Ljava/util/Base64 解析失败 未找到类“java.util.Base64”、ArrayList al = new ArrayList（）;有什么区别？和ArrayList al = new ArrayList（0）？、ArrayList 的区别 a = new ArrayList<>();和 List a = new ArrayList<>();、Cause: java.lang.IllegalArgumentException: invalid comparison: java.util.ArrayList and java.lang....的有价值信息。

• java.util.ArrayList（java.util.arraylist cannot be）
• 'java.lang.NoClassDefFoundError 由 java.lang.ClassNotFoundException 引起的 Ljava/util/Base64 解析失败 未找到类“java.util.Base64”
• ArrayList al = new ArrayList（）;有什么区别？和ArrayList al = new ArrayList（0）？
• ArrayList 的区别 a = new ArrayList<>();和 List a = new ArrayList<>();
• Cause: java.lang.IllegalArgumentException: invalid comparison: java.util.ArrayList and java.lang....

java.util.ArrayList（java.util.arraylist cannot be）

1、实现 RandomAccess 接口，建议采用 fori 循环，效率要比迭代 Iterator 要高。

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable

2、ArrayList 的底层为数组

private static final int DEFAULT_CAPACITY = 10; //默认的数组大小

private static final Object[] EMPTY_ELEMENTDATA = {}; //空的数组

private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; //默认长度的数组

transient Object[] elementData; // 被transient的属性不会被序列化，字段的生命周期仅存于调用者的内存中而不会写到磁盘里持久化

3、size 表示 list 包含元素的个数

/**
 * The size of the ArrayList (the number of elements it contains).
 *
 * @serial
 */
private int size;

4、有参构造函数，指定 list 长度，如果 initialCapacity>0, 那么生成指定长度的数组，initialCapacity==0，生成默认的空数组，initialCapacity<0, 抛出 IllegalArgumentException 异常

public ArrayList(int initialCapacity) {
    if (initialCapacity > 0) {
        this.elementData = new Object[initialCapacity];
    } else if (initialCapacity == 0) {
        this.elementData = EMPTY_ELEMENTDATA;
    } else {
        throw new IllegalArgumentException("Illegal Capacity: "+
                                           initialCapacity);
    }
}

5、无参构造函数，默认数组长度为十

/**
 * Constructs an empty list with an initial capacity of ten.
 */
public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

6、如果 c 元素不为空，生成内容为 c 的 list

public ArrayList(Collection<? extends E> c) {
    elementData = c.toArray();
    if ((size = elementData.length) != 0) {
        // c.toArray might (incorrectly) not return Object[] (see 6260652) 
        if (elementData.getClass() != Object[].class)   //原因参考：https://www.cnblogs.com/cmdra/p/5901793.html
            elementData = Arrays.copyOf(elementData, size, Object[].class);
    } else {
        // replace with empty array.
        this.elementData = EMPTY_ELEMENTDATA;
    }
}

7、举例：当一个默认长度为 10 的 ArrayList，又增加一个元素时，动态增长内存长度为 15，该方法就是通过 Arrays.copyOf (elementData, size) 生成指定长度的 list 去除多余内存长度

/**
 * Trims the capacity of this <tt>ArrayList</tt> instance to be the
 * list''s current size.  An application can use this operation to minimize
 * the storage of an <tt>ArrayList</tt> instance.
 */
public void trimToSize() {
    modCount++;
    if (size < elementData.length) {
        elementData = (size == 0)
          ? EMPTY_ELEMENTDATA
          : Arrays.copyOf(elementData, size);
    }
}

8、get (index) 首先校验是否数组越界，然后返回 elementData [index]

public E get(int index) {
    rangeCheck(index);

    return elementData(index);
}

9、set(int,E)    

public E set(int index, E element) {
    rangeCheck(index);

    E oldValue = elementData(index);
    elementData[index] = element;
    return oldValue;
}

10、add (E e) 方法

public boolean add(E e) {
    ensureCapacityInternal(size + 1);  // Increments modCount!! 提前校验增长后长度，有必要的话先预增长
    elementData[size++] = e;
    return true;
}

private void ensureCapacityInternal(int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {  //如果elementData是默认长度10的空list
        minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);        // minCapacity 取两者中的较大值 
    }
    ensureExplicitCapacity(minCapacity);
}

private void ensureExplicitCapacity(int minCapacity) {
    modCount++;

    // overflow-conscious code
    if (minCapacity - elementData.length > 0)  //判断增加元素后的长度是否大于现在的arraylist的创建大小，大于的话就增长
        grow(minCapacity);
}

private void grow(int minCapacity) {
    // overflow-conscious code
    int oldCapacity = elementData.length;
    int newCapacity = oldCapacity + (oldCapacity >> 1);   //新创建的数组长度为原来的1.5倍
    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);
}

11、E remove (int index) 还有一个 fastmove 的方法，不同点在于去除了 rangeCheck（index）的判断

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;
}

12、 截取，左闭右开

public List<E> subList(int fromIndex, int toIndex) {
    subListRangeCheck(fromIndex, toIndex, size);
    return new SubList(this, 0, fromIndex, toIndex);
}

13、removeIf (Predicate<? super E> filter)  自定义一个 CommonPredicate 类实现 Predicate<T > 接口，定义一个属性 varc1，实现 test 接口，重新 test 逻辑，然后使用的时候定义 CommonPredicate.varc1 值为你想要筛选的条件，就可以达到筛选的效果

@Override
public boolean removeIf(Predicate<? super E> filter) {
    Objects.requireNonNull(filter);
    // figure out which elements are to be removed
    // any exception thrown from the filter predicate at this stage
    // will leave the collection unmodified
    int removeCount = 0;
    final BitSet removeSet = new BitSet(size);
    final int expectedModCount = modCount;
    final int size = this.size;
    for (int i=0; modCount == expectedModCount && i < size; i++) {
        @SuppressWarnings("unchecked")
        final E element = (E) elementData[i];
        if (filter.test(element)) {
            removeSet.set(i);
            removeCount++;
        }
    }
    if (modCount != expectedModCount) {
        throw new ConcurrentModificationException();
    }

    // shift surviving elements left over the spaces left by removed elements
    final boolean anyToRemove = removeCount > 0;
    if (anyToRemove) {
        final int newSize = size - removeCount;
        for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {
            i = removeSet.nextClearBit(i);
            elementData[j] = elementData[i];
        }
        for (int k=newSize; k < size; k++) {
            elementData[k] = null;  // Let gc do its work
        }
        this.size = newSize;
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
        modCount++;
    }

    return anyToRemove;
}

14、replaceAll (UnaryOperator<E> operator)  同上，可以根据自己的要求重写替换逻辑

@Override
@SuppressWarnings("unchecked")
public void replaceAll(UnaryOperator<E> operator) {
    Objects.requireNonNull(operator);
    final int expectedModCount = modCount;
    final int size = this.size;
    for (int i=0; modCount == expectedModCount && i < size; i++) {
        elementData[i] = operator.apply((E) elementData[i]);
    }
    if (modCount != expectedModCount) {
        throw new ConcurrentModificationException();
    }
    modCount++;
}

//列子
public class CommonPredicate<T> implements Predicate<T> {

    T filter;

    @Override
    public boolean test(T t) {
        if (filter.equals( t )) {
            return true;
        } else {
            return false;
        }
    }

}

public class ArrayListTest {

    private static CommonPredicate<Object> predicate = new CommonPredicate<>();

    private static CommonUnaryOperator<Object> unaryOperator = new CommonUnaryOperator<>();

    public static void main(String[] args) {

        predicate.filter = (2);
        unaryOperator.varc1=(2);

        ArrayList<Object> list1 = new ArrayList<>();
        list1.add( 1 );
        list1.add( 2 );
        list1.add( 3 );
        list1.add( 2 );

        //list1.removeIf( predicate ); //打印出来的结果是1 3
        list1.replaceAll( unaryOperator ); //打印出来的结果是2 99 2 99
        list1.forEach( o -> System.out.println( o ) );
    }
}

15、sort (Comparator<? super E> c)，见下面例子

@Override
@SuppressWarnings("unchecked")
public void sort(Comparator<? super E> c) {
    final int expectedModCount = modCount;
    Arrays.sort((E[]) elementData, 0, size, c);
    if (modCount != expectedModCount) {
        throw new ConcurrentModificationException();
    }
    modCount++;
}

---------------------例子----------------------
@Data
public class Student {
    String name;
    Integer id;

    public Student(Integer id,String name) {
        this.name = name;
        this.id = id;
    }
}

public class StudentComparator implements Comparator<Student> {

    @Override
    public int compare(Student o1, Student o2) {
        return o1.id.compareTo( o2.id );
    }
}

public class SortTest {


    public static void main(String[] args) {

        StudentComparator studentComparator = new StudentComparator();

        List<Student> myList = new ArrayList<>();

        myList.add( new Student( 4, "David" ) );
        myList.add( new Student( 2, "Tom" ) );
        myList.add( new Student( 5, "Rohit" ) );
        myList.add( new Student( 1, "Paul" ) );
        myList.add( new Student( 3, "Vishal" ) );

        myList.sort( studentComparator );

        System.out.println( myList );

    }
}    打印结果：[Student(name=Paul, id=1), Student(name=Tom, id=2), Student(name=Vishal, id=3), Student(name=David, id=4), Student(name=Rohit, id=5)]

'java.lang.NoClassDefFoundError 由 java.lang.ClassNotFoundException 引起的 Ljava/util/Base64 解析失败 未找到类“java.util.Base64”

List<Integer> simpleList = new ArrayList<>();
System.out.println(simpleList.getClass().getName());
// java.util.ArrayList

List<Integer> a = new ArrayList<>();

var a = new ArrayList<Integer>();

<choose>
      <when test="names!= null and names.size!=''''">
        and name in
        <foreach collection="names" item="name" index="index" open="(" close=")" separator=",">
          #{name}
        </foreach>
      </when>
      <otherwise>
        and name= ''''
      </otherwise>
    </choose>

<choose>
      <when test="names!= null and names.size>0">
        and name in
        <foreach collection="names" item="name" index="index" open="(" close=")" separator=",">
          #{name}
        </foreach>
      </when>
      <otherwise>
        and name= ''''
      </otherwise>
    </choose>