12.26 exercise summary

1、 Traversing a one-dimensional array --for each

package test;
public class Test01 {
    public static void main(String[] args) {
        int ef[]={1,2,3,4,5,6};
        for(int a:ef){
            System.out.print(a);//123456
        }
    }
}

Be careful :for(int a:ef) Things represented in .

2、Arrays class -- Copy （arraycopy), Sort (Arrays.sort), lookup (Arrays.binarysearch), fill (Arrays.fill)

package test;

import java.util.Arrays;

public class Test02 {
    public static void main(String[] args) {
        int a[]={0,0,0,0,0,0};
        int b[]={1,2,3};
        System.arraycopy(a,1,b,1,2);
        for(int c:b){
            System.out.print(c);//100
        }
        System.out.println();
        int A[]={1,3,2,4,6,5};
        Arrays.sort(A);
        for(int m:A){
            System.out.println(m);//123456
        }
        System.out.println(Arrays.toString(A));//[1, 2, 3, 4, 5, 6]
        System.out.println(Arrays.binarySearch(A,2));//1
        System.out.println(Arrays.binarySearch(A,0));//-1  No return found -1
        Arrays.fill(A,2,3,100);
        System.out.println(Arrays.toString(A));//[1, 2, 100, 4, 5, 6]
    }
}

Be careful ：1 Copy is to a Copy the contents to b in , Don't go against the target . namely arraycopy Method is the copied object , The second array is the copy object .2 binarySearch() It uses dichotomy to find the position of the element , If the index of the returned element is found , If you don't find it, go back -1.3 Arrays.fill() Execute before exporting , Direct output will report an exception . explain Array.fill(A,2,3,100): Yes A Fill in the elements in , Subscript to be 2-3, But not including 3, The filling element is 100.

2、Object An array stores table data

import java.util.Arrays;

public class Test03 {
    public static void main(String[] args) {
        Object a1[]={1," full name "," Gender "," Age "};
        Object a2[]={2," Zhang San "," male ",10};
        Object a3[]={3," Li Si "," Woman ",18};
        System.out.println(Arrays.toString(a1));
        System.out.println(Arrays.toString(a2));
        System.out.println(Arrays.toString(a3));
    }
}

Object Arrays can hold different types of content

3、javabean And array to save table data

package test;

import java.util.Arrays;

public class Test04 {
    public static void main(String[] args) {
        Bean[] bean = {
                new Bean(" Xiaohong ", 18, " Casting engineer ", 10000),
                new Bean(" Xiao Ming ", 21, " Software engineer ", 10000),
                new Bean(" floret ", 23, " Hardware Engineer ", 10000)
        };
        System.out.println(Arrays.toString(bean));//[ Xiaohong 	18	 Casting engineer 	10000,  Xiao Ming 	21	 Software engineer 	10000,  floret 	23	 Hardware Engineer 	10000]
        for(Bean b:bean){
            System.out.println(b);
        }
        // Xiaohong 	18	 Casting engineer 	10000
        // Xiao Ming 	21	 Software engineer 	10000
        // floret 	23	 Hardware Engineer 	10000
    }
}
class Bean{
    private String name;
    private int age;
    private String job;
    private int salary;
    public  Bean(String name,int age,String job,int salary){
        this.name=name;
        this.age=age;
        this.job=job;
        this.salary=salary;
    }
    public String toString(){
        return name+"\t"+age+"\t"+job+"\t"+salary;
    }
}

Be careful ：class Bean Cannot create on class Test04 in , If you want to 1 Output what you want , Need to be right toString Method ,Javabean What does that mean? , Where does it reflect ？

3、 Sorting of objects --Comparable Interface

package test;

import java.util.Arrays;

public class Test05 {
    public static void main(String[] args) {
        Com[] com={
                new Com(1," Zhang San "," Woman "," building engineer "),
                new Com(3," Wang Wu "," male "," Casting engineer "),
                new Com(2," Li Si "," male "," Software engineer ")
        };
        for(Com c:com){
            System.out.println(c);
        }
        //1	 Zhang San 	 Woman 	 building engineer 
        //3	 Wang Wu 	 male 	 Casting engineer 
        //2	 Li Si 	 male 	 Software engineer 
        Arrays.sort(com);
        for(Com c:com){
            System.out.println(c);
        }
        //1	 Zhang San 	 Woman 	 building engineer 
        //2	 Li Si 	 male 	 Software engineer 
        //3	 Wang Wu 	 male 	 Casting engineer 
    }
}
class Com implements Comparable{
    private int number;
    private String name;
    private String gender;
    private String job;
    public String toString(){
        return number+"\t"+name+"\t"+gender+"\t"+job;
    }
    public Com(int number,String name,String gender,String job){
        super();
        this.number=number;
        this.name=name;
        this.gender=gender;
        this.job=job;
    }
    public int compareTo(Object o){
        Com com=(Com)o;
        if(this.number<com.number){
            return -1;
        }
        if(this.number>com.number){
            return 1;
        }
        return 0;
    }
}

stay javabean On the basis of , There are more things about sorting , To implement sorting, you need to call Arrays.sort(); Class needs to inherit Comparable Interface , Therefore, we need to add super(); And to compareTo Method .

4、 Bubble sorting and optimization

package test;

import java.util.Arrays;

public class Test06 {
    public static void main(String[] args) {
        int[] a={1,4,2,5,8,9,7};
        int temp;
        for(int i=0;i<a.length;i++){
            for(int j=0;j<a.length-1-i;j++){
                if (a[j]>a[j+1]){
                    temp=a[j];
                    a[j]=a[j+1];
                    a[j+1]=temp;
                }
                }
            }
        System.out.println(Arrays.toString(a));
        }
    }

  Be careful ：j<a.length-1-i; If you don't -1, There will be ArrayIndexOutOfBoundsException abnormal .

package test;

import java.util.Arrays;

public class Test06 {
    public static void main(String[] args) {
        int[] a={1,4,2,5,8,9,7};
        int temp=0;
        for(int i=0;i<a.length;i++){
            boolean flag=true;
            for(int j=0;j<a.length-1-i;j++){
                if (a[j]>a[j+1]){
                    temp=a[j];
                    a[j]=a[j+1];
                    a[j+1]=temp;
                    flag=false;
                }
                }
                if(flag){
                    break;
                }
            }
        System.out.println(Arrays.toString(a));
        }
    }

The above code optimizes bubble sorting , Avoid repeated comparisons , it is to be noted that boolean flag=true Wait in for The position of the loop , If the position is incorrect , You can't achieve your goal ！ 

5、 Dichotomy to find elements in an array

package test;

import java.util.Arrays;

public class Test07 {
    public static void main(String[] args) {
        int arr[]={1,4,6,8,7,9,2,3};
        Arrays.sort(arr);
        System.out.println(Arrays.toString(arr));
        System.out.println(binarySearch(arr,4));
    }
    public static int binarySearch(int array[],int value){
        int low=0;
        int high= array.length-1;
        while(low<=high){
            int middle=(low+high)/2;
            if(value<middle){
                high=middle-1;
            }
            else if(value>middle){
                low=middle+1;
            }else{
                return middle;
            }
        }
        return -1;
    }
}

Be careful ：return -1 The location of ,int middle=(low+high)/2 Need to change every time .