TestArrays

package samples;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;

public class ListArrays {
	public static void main(String[] args) {
		List<User> lUser = User.initUsers();

		/*
		 * convert list to arrays
		 */
		User[] arrUsers = new User[lUser.size()];
		arrUsers = lUser.toArray(arrUsers);

		
		/*
		 * convert array to list 
		 */
		List<User> lUser2 = Arrays.asList(arrUsers);

		/*
		 * Sort Array
		 */
		Arrays.sort(arrUsers, new AgeComparator());
		/*
		 * 按照年龄来找而已 如果多个符合条件,也只返回其中一个
		 */
		System.out.println(Arrays.binarySearch(arrUsers, new User("赵倩", 30),
				new AgeComparator()));

		populateArray(arrUsers); 
		populateCollection(lUser2);
		/*
		 * Copy都是softCopy,not DeepCopy
		 * 
		 */

		System.out.println(Arrays.deepToString(arrUsers));
		System.out.println(arrUsers.toString());
		System.out.println(Arrays.deepHashCode(arrUsers));
		System.out.println(arrUsers.hashCode());

		System.out.println("------------------------>Arrays.equals");
		
		/*
		 * 基本类型
		 */
		int[] a = { 1, 2 };
		int[] b = { 1, 2 };
		System.out.println(a.equals(b));// F do not use this method!
		System.out.println(Arrays.equals(a, b)); // T a[0]==b[0]? a[1]==b[1]?
													// 用等好比较,基本类型
		/*
		 * compile error
		 */
		// System.out.println(Arrays.deepEquals(a, b));
		int[][] c = { { 1, 2 }, { 1, 2 } };
		int[][] d = { { 1, 2 }, { 1, 2 } };
		System.out.println(c.equals(d));// F do not use this method!
		System.out.println(Arrays.equals(c, d)); // F
		System.out.println(Arrays.deepEquals(c, d));// T //unlimited loop

		
		/*
		 * Object 
		 * Arrays.eqalas 适合一维数组
		 * Arrays.deepequals 多维
		 */
		User u1 = new User("张三", 25);
		User u2 = new User("李四", 26);
		User u3 = new User("王五", 21);

		User[] arrUsers2 = { u1, u2, u3 };
		System.out.println(arrUsers.equals(arrUsers2));// F
		System.out.println(Arrays.equals(arrUsers, arrUsers2));// T  一定要重写equals
		System.out.println(Arrays.deepEquals(arrUsers, arrUsers2));// T 
		
		User[][] arrUsersDeep1 = {{u1,u2}};
		User[][] arrUsersDeep2 = {{u1,u2}};
		System.out.println(arrUsersDeep1.equals(arrUsersDeep2));// F
		System.out.println(Arrays.equals(arrUsersDeep1, arrUsersDeep2));// F  一定要重写equals ,one dimension array
		System.out.println(Arrays.deepEquals(arrUsersDeep1, arrUsersDeep2));// T   //unlimited loop

	}

	public static void populateArray(User[] arrUsers) {
		for (User user : arrUsers) {
			System.out.print(user.getName());
			System.out.print(user.getAge() + " ");
		}
		System.out.println();
	}

	public static void populateCollection(Collection<User> c) {
		Iterator<User> iter = c.iterator();
		while (iter.hasNext()) {
			User user = iter.next();
			System.out.print(user.getName());
			System.out.print(user.getAge() + " ");
		}
		System.out.println();
	}
}

class User implements Comparable<User> {
	private String name;

	private int age;

	public User(String name, int age) {
		this.name = name;
		this.age = age;
	}

	/*
	 * return List<User>
	 */
	public static List<User> initUsers() {
		List<User> l = new ArrayList<User>();
		l.add(new User("张三", 25));
		l.add(new User("李四", 26));
		l.add(new User("王五", 21));

		return l;
	}

	/*
	 * 
	 * 导致的结果就是所有的对象都一致 TreeSet,TreeMap来说,根据compareTo来判定是否是同一个对象 或者 Map的key
	 * ts.size();永远是1,不管放入多少对象
	 */
	public int compareTo(User u) {
		return 0;
	}

	public int getAge() {
		return age;
	}

	public void setAge(int age) {
		this.age = age;
	}

	public String getName() {

		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	@Override
	public boolean equals(Object o) {
		if (o instanceof User) {
			return true;
		}
		return false;
	}
}

class AgeComparator implements Comparator<User> {
	public int compare(User u1, User u2) {
		return u1.getAge() - u2.getAge();
	}
}

 

 

http://hi.baidu.com/happy19840402/blog/item/3b12a610b45353f9c3ce7984.html

List.toArray,因为数组要长度,所以麻烦点

 

 

 

List list = new ArrayList();
   list.add(new Person());
   list.add(new Person());
   list.add(new Person());
   Person[] person = (Person[]) list.toArray();

运行程序会抛出异常：java.lang.ClassCastException

但运行如下代码：

List list = new ArrayList();
   list.add(new Person());
   list.add(new Person());
   list.add(new Person());
   Person[] person = (Person[]) list.toArray(new Person[0]);

程序正确执行，

原因，先看ArrayList的两个toArray()方法的源代码：

public Object[] toArray() {
    Object[] result = new Object[size];
    System.arraycopy(elementData, 0, result, 0, size);
    return result;
}


public Object[] toArray(Object a[]) {
    if (a.length < size)
        a = (Object[])java.lang.reflect.Array.newInstance(a.getClass().getComponentType(), size);
    System.arraycopy(elementData, 0, a, 0, size);
    if (a.length > size)
        a[size] = null;
    return a;
}

可以看出，不带参数的toArray方法，是构造的一个Object数组，然后进行数据拷贝，此时进行转型就会产生ClassCastException而带参数的toArray方法，则是根据参数数组的类型，构造了一个对应类型的，长度跟ArrayList的size一致的空数组，虽然方法本身还是以Object数组的形式返回结果，不过由于构造数组使用的ComponentType跟需要转型的ComponentType一致，就不会产生转型异常。

那么，对于不带参数的toArray方法，它有什么特殊意义呢？还是用代码说话：

ArrayList list = new ArrayList();
list.add(new Person());
list.add(new Person());
list.add(new Person());//这里不需要转型，也不能使用转型
Object[] ss = list.toArray();
//这里可以进行转型，取出原ArrayList里面存放的对象
for (int i = 0; i < ss.length; i++) {
Person person= (Person) ss[i];System.out.println(person);
}

 

Arrays:
asList
binarySearch
equals
fill
hashCode
sort
toString

deepEquals(Object[] a1, Object[] a2)
deepHashCode(Object[] a)
deepToString(Object[] a)