Java常用API(三)
9.5 數(shù)組工具類
9.5.1 java.util.Arrays類
java.util.Arrays數(shù)組工具類，提供了很多靜態(tài)方法來(lái)對(duì)數(shù)組進(jìn)行操作，而且如下每一個(gè)方法都有各種重載形式，以下只列出int[]和Object[]類型的，其他類型的數(shù)組依次類推：
- 數(shù)組元素拼接
- static String toString(int[] a) ：字符串表示形式由數(shù)組的元素列表組成，括在方括號(hào)（"[]"）中。相鄰元素用字符 ", "（逗號(hào)加空格）分隔。形式為：[元素1，元素2，元素3。。。]
- static String toString(Object[] a) ：字符串表示形式由數(shù)組的元素列表組成，括在方括號(hào)（"[]"）中。相鄰元素用字符 ", "（逗號(hào)加空格）分隔。元素將自動(dòng)調(diào)用自己從Object繼承的toString方法將對(duì)象轉(zhuǎn)為字符串進(jìn)行拼接，如果沒(méi)有重寫，則返回類型@hash值，如果重寫則按重寫返回的字符串進(jìn)行拼接。
- 數(shù)組排序
- static void sort(int[] a) ：將a數(shù)組按照從小到大進(jìn)行排序
- static void sort(int[] a, int fromIndex, int toIndex) ：將a數(shù)組的[fromIndex, toIndex)部分按照升序排列
- 數(shù)組元素的二分查找
- static int binarySearch(int[] a, int key)
- 數(shù)組的復(fù)制
- static int[] copyOf(int[] original, int newLength) ：根據(jù)original原數(shù)組復(fù)制一個(gè)長(zhǎng)度為newLength的新數(shù)組，并返回新數(shù)組
- static <T> T[] copyOf(T[] original,int newLength)：根據(jù)original原數(shù)組復(fù)制一個(gè)長(zhǎng)度為newLength的新數(shù)組，并返回新數(shù)組
- static int[] copyOfRange(int[] original, int from, int to) ：復(fù)制original原數(shù)組的[from,to)構(gòu)成新數(shù)組，并返回新數(shù)組
- static <T> T[] copyOfRange(T[] original,int from,int to)：復(fù)制original原數(shù)組的[from,to)構(gòu)成新數(shù)組，并返回新數(shù)組
- 比較兩個(gè)數(shù)組是否相等
- static boolean equals(int[] a, int[] a2) ：比較兩個(gè)數(shù)組的長(zhǎng)度、元素是否完全相同
- static boolean equals(Object[] a,Object[] a2)：比較兩個(gè)數(shù)組的長(zhǎng)度、元素是否完全相同
- 填充數(shù)組
- static void fill(int[] a, int val) ：用val值填充整個(gè)a數(shù)組
- static void fill(Object[] a,Object val)：用val對(duì)象填充整個(gè)a數(shù)組
- static void fill(int[] a, int fromIndex, int toIndex, int val)：將a數(shù)組[fromIndex,toIndex)部分填充為val值
- static void fill(Object[] a, int fromIndex, int toIndex, Object val) ：將a數(shù)組[fromIndex,toIndex)部分填充為val對(duì)象
import org.junit.Test;
import java.text.Collator;
import java.util.Arrays;
import java.util.Comparator;
import java.util.Locale;
public class TestArrays {
@Test
public void test01() {
int[] arr = {1, 2, 3, 4, 5};
System.out.println(Arrays.toString(arr));
Student[] students = new Student[3];
students[0] = new Student("張三", 96);
students[1] = new Student("李四", 85);
students[2] = new Student("王五", 98);
System.out.println(Arrays.toString(students));
}
@Test
public void test02() {
int[] arr = {3, 2, 5, 1, 6};
System.out.println("排序前" + Arrays.toString(arr));
Arrays.sort(arr);
System.out.println("排序后" + Arrays.toString(arr));
Student[] students = new Student[3];
students[0] = new Student("張三", 96);
students[1] = new Student("李四", 85);
students[2] = new Student("王五", 98);
System.out.println(Arrays.toString(students));
Arrays.sort(students);
System.out.println(Arrays.toString(students));
Arrays.sort(students, new Comparator() {
@Override
public int compare(Object o1, Object o2) {
return Collator.getInstance(Locale.CHINA).compare(((Student) o1).getName(), ((Student) o2).getName());
}
});
System.out.println(Arrays.toString(students));
}
@Test
public void test03() {
int[] arr1 = {1, 2, 3, 4, 5};
int[] arr2 = {1, 2, 3, 4, 5};
System.out.println(Arrays.equals(arr1, arr2));
}
@Test
public void test04() {
int[] arr1 = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
int[] arr2 = Arrays.copyOf(arr1, 5);
int[] arr3 = Arrays.copyOfRange(arr1, 3, 8);
System.out.println(Arrays.toString(arr2));
System.out.println(Arrays.toString(arr3));
Arrays.fill(arr1, 5, 9, 3);
System.out.println(Arrays.toString(arr1));
}
}
package com.atguigu.arrays;
public class Student implements Comparable {
private String name;
private int score;
public Student(String name, int score) {
this.name = name;
this.score = score;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getScore() {
return score;
}
public void setScore(int score) {
this.score = score;
}
@Override
public String toString() {
return "Student{" +
"name='" + name + '\'' +
", score=" + score +
'}';
}
@Override
public int compareTo(Object o) {
return this.score - ((Student)o).score;
}
}
9.5.2 java.lang.System類
系統(tǒng)類中很多好用的方法，其中幾個(gè)如下：
static void arraycopy(Object src, int srcPos, Object dest, int destPos, int length)：
從指定源數(shù)組中復(fù)制一個(gè)數(shù)組，復(fù)制從指定的位置開(kāi)始，到目標(biāo)數(shù)組的指定位置結(jié)束。常用于數(shù)組的插入和刪除
import org.junit.Test;
import java.util.Arrays;
public class TestSystemArrayCopy {
@Test
public void test01(){
int[] arr1 = {1,2,3,4,5};
int[] arr2 = new int[10];
System.arraycopy(arr1,0,arr2,3,arr1.length);
System.out.println(Arrays.toString(arr1));
System.out.println(Arrays.toString(arr2));
}
@Test
public void test02(){
int[] arr = {1,2,3,4,5};
System.arraycopy(arr,0,arr,1,arr.length-1);
System.out.println(Arrays.toString(arr));
}
@Test
public void test03(){
int[] arr = {1,2,3,4,5};
System.arraycopy(arr,1,arr,0,arr.length-1);
System.out.println(Arrays.toString(arr));
}
}
9.6 比較器
9.6.1 java.lang.Comparable自然比較器接口
我們知道基本數(shù)據(jù)類型的數(shù)據(jù)（除boolean類型外）需要比較大小的話，之間使用比較運(yùn)算符即可，但是引用數(shù)據(jù)類型是不能直接使用比較運(yùn)算符來(lái)比較大小的。那么，如何解決這個(gè)問(wèn)題呢？
Java給所有引用數(shù)據(jù)類型的大小比較，指定了一個(gè)標(biāo)準(zhǔn)接口，就是java.lang.Comparable接口：
package java.lang;
public interface Comparable{
int compareTo(Object obj);
}
那么我們想要使得我們某個(gè)類的對(duì)象可以比較大小，怎么做呢？步驟：
第一步：哪個(gè)類的對(duì)象要比較大小，哪個(gè)類就實(shí)現(xiàn)java.lang.Comparable接口，并重寫方法
方法體就是你要如何比較當(dāng)前對(duì)象和指定的另一個(gè)對(duì)象的大小
- 第二步：對(duì)象比較大小時(shí)，通過(guò)對(duì)象調(diào)用compareTo方法，根據(jù)方法的返回值決定誰(shuí)大誰(shuí)小。
- this對(duì)象（調(diào)用compareTo方法的對(duì)象）大于指定對(duì)象（傳入compareTo()的參數(shù)對(duì)象）返回正整數(shù)
- this對(duì)象（調(diào)用compareTo方法的對(duì)象）小于指定對(duì)象（傳入compareTo()的參數(shù)對(duì)象）返回負(fù)整數(shù)
- this對(duì)象（調(diào)用compareTo方法的對(duì)象）等于指定對(duì)象（傳入compareTo()的參數(shù)對(duì)象）返回零
代碼示例：
package com.atguigu.array;
public class Student implements Comparable {
private int id;
private String name;
private int score;
private int age;
public Student(int id, String name, int score, int age) {
this.id = id;
this.name = name;
this.score = score;
this.age = age;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getScore() {
return score;
}
public void setScore(int score) {
this.score = score;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
@Override
public String toString() {
return "Student{" +
"id=" + id +
", name='" + name + '\'' +
", score=" + score +
", age=" + age +
'}';
}
@Override
public int compareTo(Object o) {
//這些需要強(qiáng)制，將o對(duì)象向下轉(zhuǎn)型為Student類型的變量，才能調(diào)用Student類中的屬性
//默認(rèn)按照學(xué)號(hào)比較大小
Student stu = (Student) o;
return this.id - stu.id;
}
}
package com.atguigu.array;
public class TestComparable {
public static void main(String[] args) {
Student[] arr = new Student[5];
arr[0] = new Student(3,"張三",90,23);
arr[1] = new Student(1,"熊大",100,22);
arr[2] = new Student(5,"王五",75,25);
arr[3] = new Student(4,"李四",85,24);
arr[4] = new Student(2,"熊二",85,18);
//單獨(dú)比較兩個(gè)對(duì)象
System.out.println(arr[0].compareTo(arr[1]));
System.out.println(arr[1].compareTo(arr[2]));
System.out.println(arr[2].compareTo(arr[2]));
System.out.println("所有學(xué)生：");
for (int i = 0; i < arr.length; i++) {
System.out.println(arr[i]);
}
System.out.println("按照學(xué)號(hào)排序：");
for (int i = 1; i < arr.length; i++) {
for (int j = 0; j < arr.length-i; j++) {
if(arr[j].compareTo(arr[j+1])>0){
Student temp = arr[j];
arr[j] = arr[j+1];
arr[j+1] = temp;
}
}
}
for (int i = 0; i < arr.length; i++) {
System.out.println(arr[i]);
}
}
}
