第四章 数组

4.1 概念

数组是一个变量，存储相同数据类型的一组数据，java中数组是长度是固定的，下标是从0开始的，数组会自动初始。

package cn;

public class Arr1 {

	public static void main(String[]  args) {
		int[] ns = new int[3];//0 1 2
		ns[ns.length-1] = 10;//异常 下标越界
		System.out.println(ns[0]);//0
		System.out.println(ns[1]);//0
		System.out.println(ns[2]);//10
		
		boolean[] fs = new boolean[3];
		System.out.println(fs[0]);//false
		
		String[] ss = new String[] {"hello","java","javascript"};
		System.out.println(ss.length);//3
		System.out.println(ss[2]);//javascript
		
		String s2[] = {"aa","cc","dd","ee"};
		String[] s3 = new String[6];
		System.out.println(s3[1]);//null
		
		//二维数组
		double[][] ds = new double[][] {{10,20,30},{60,70,80},{11,22,33,44,55,66}};
		
		//System.out.println(ds[1][5]);//异常错误
		System.out.println(ds[2][5]);//66.0

	}

}

package cn;

import java.util.Date;
import java.util.Scanner;

public class Arr2 {

	public static void main(String[] args) {
		// java 中 java.lang.Object 此类为所有类的父类，（基类）
		Object[] os = { 1, 2.0, "java", 'c', true, new Date(), new Scanner(System.in) };

		Scanner sc = (Scanner) os[os.length - 1];
		int t = sc.nextInt();
		System.out.println(t);

	}

}

• 数组遍历

  package cn;
  
  import java.util.Arrays;
  import java.util.Random;
  
  public class Arr3 {
  
  	public static void main(String[] args) {
  		int[] ns = new int[10];
  		Random rand = new Random();
  		for(int i=0;i b - a); // 降序
  		System.out.println(Arrays.toString(n2));
  
  		int a = 10;
  		Integer b = 20;
  
  		Integer[] cc = new Integer[nn.length];
  		for (int i = 0; i  y - x);
  		System.out.println(Arrays.toString(cc));
  
  	}
  
  }
  
  

  Student.java 类

  package cn;
  
  public class Student {
  	private String sno;
  	private String sname;
  	private int score;
  	public String getSno() {
  		return sno;
  	}
  	public void setSno(String sno) {
  		this.sno = sno;
  	}
  	public String getSname() {
  		return sname;
  	}
  	public void setSname(String sname) {
  		this.sname = sname;
  	}
  	public int getScore() {
  		return score;
  	}
  	public void setScore(int score) {
  		this.score = score;
  	}
  	public Student(String sno, String sname, int score) {
  		super();
  		this.sno = sno;
  		this.sname = sname;
  		this.score = score;
  	}
  	public Student() {
  		super();
  		// TODO Auto-generated constructor stub
  	}
  	@Override
  	public String toString() {
  		return "Student [sno=" + sno + ", sname=" + sname + ", score=" + score + "]";
  	}
  	
  }
  
  

  排序，升序降序

  package cn;
  
  import java.util.Arrays;
  
  public class Arr6 {
  
  	public static void main(String[] args) {
  		String[] ls = { "python", "c", "go", "perl", "php", "java", "javascript", "html", "css", "mysql" };
  		System.out.println(Arrays.toString(ls));
  		Arrays.sort(ls);// 升序
  		System.out.println(Arrays.toString(ls));
  		Arrays.sort(ls, (a, b) -> b.compareTo(a));// 自然排序 降序
  		System.out.println(Arrays.toString(ls));
  
  		Arrays.sort(ls, (a, b) -> a.length() - b.length());// 按字符个数，升序
  		System.out.println(Arrays.toString(ls));
  		Arrays.sort(ls, (a, b) -> b.length() - a.length());// 按字符个数，降序
  		System.out.println(Arrays.toString(ls));
  
  		Student[] sts = new Student[3];
  		sts[0] = new Student("10","李四",80);
  		sts[1] = new Student("11","张三丰",30);
  		sts[2] = new Student("12","赵强",90);
  		System.out.println(Arrays.toString(sts));
  		Arrays.sort(sts,(a,b)->a.getScore()-b.getScore());
  		System.out.println(Arrays.toString(sts));
  		Arrays.sort(sts,(a,b)->b.getScore()-a.getScore());
  		System.out.println(Arrays.toString(sts));
  	}
  
  }
  
  

  4.3 数组排序

  面试题：声明10（1-100)随机整数的数组，遍历输出，冒泡排序，再次遍历输出。

  package cn;
  
  import java.util.Arrays;
  import java.util.Random;
  
  public class Arr7 {
  
  	public static void main(String[] args) {
  		int[] num = new int[10];
  		System.out.println(Arrays.toString(num));
  		// 赋值
  		Random rand = new Random();
  		for (int i = 0; i