Use of arrays tool class

This class contains various methods for manipulating arrays （ Such as sorting and searching ）.

Common methods

• Convert an array to a linked list

public static <T> List<T> asList(T... a)

import java.util.Arrays;
import java.util.List;
public class Test {
    
	public static void main(String[] args) {
    
		String[] str = {
    "abc", "def", "ghi", "jkl"};
		List<String> list = Arrays.asList(str);
		for (String string : list) {
    
			System.out.println(string);
		}
	}
}

abc
def
ghi
jkl

• Two points search

public static int binarySearch(int[] a, 
                               int key)
// Array types can be replaced with other types 

The premise of binary search is that the array must be sorted

import java.util.Arrays;
public class Test {
    
	public static void main(String[] args) {
    
		int[] arr = {
    21, 32, 41, 12, 32, 11, 10, -4};
		Arrays.sort(arr);
		System.out.println(Arrays.binarySearch(arr, 23));
		System.out.println(Arrays.binarySearch(arr, 12));
	}
}

-6
3

• Copy the array

public static int[] copyOf(int[] original,
                           int newLength);
public static int[] copyOfRange(int[] original,
                                int from,
                                int to)

import java.util.Arrays;
public class Test {
    
	public static void main(String[] args) {
    
		int[] arr = {
    21, 33, 41, 12, 32, 11, 10, -4};
		int[] copy = Arrays.copyOf(arr, arr.length - 2);
		int[] copy1 = Arrays.copyOfRange(arr, 2, 5);
		System.out.println("copy");
		for (int i : copy) {
    
			System.out.println(i);
		}
		System.out.println("copy1");
		for (int i : copy1) {
    
			System.out.println(i);
		}
	}
}

copy
21
33
41
12
32
11
copy1
41
12
32

• Determine whether the arrays are equal

public static boolean equals(int[] a,
                             int[] a2)

import java.util.Arrays;
public class Test {
    
	public static void main(String[] args) {
    
		int[] arr = {
    21, 33, 41, 12, 32, 11, 10, -4};
		int[] copy = Arrays.copyOf(arr, arr.length);
		System.out.println(Arrays.equals(arr, copy));
	}
}

true

• Fill in the same values

public static void fill(int[] a,
                        int val)

import java.util.Arrays;
public class Test {
    
	public static void main(String[] args) {
    
		int[] arr = new int[4];
		Arrays.fill(arr, 45);
		for (int i : arr) {
    
			System.out.println(i);
		}
	}
}

45
45
45
45

The method of sorting

Arrays Sorting methods in tool classes sort Generally, it is the default sorting method , Can be a value , String, etc

import java.util.Arrays;

public class Test {
    
	public static void main(String[] args) {
    
		int[] arr = {
    21, 33, 41, 12, 32, 11, 10, -4};
		Arrays.sort(arr);
		for (int i : arr) {
    
			System.out.print(i + " ");
		}
	}
}

-4 10 11 12 21 32 33 41 

Of course , We can also customize the sorting , There are two ways

• Use Comparator The comparator

package  aggregate ;

import java.util.Arrays;
import java.util.Comparator;

public class Test {
    
	public static void main(String[] args) {
    
		Stu s1 = new Stu(" Zhang San ", 19);
		Stu s2 = new Stu(" Li Si ", 20);
		Stu s3 = new Stu(" Wang Wu ", 17);
		Stu[] s = new Stu[3];
		s[0] = s1;
		s[1] = s2;
		s[2] = s3;
		for (Stu stu : s) {
    
			System.out.println(stu);
		}
		Arrays.sort(s, new Comparator<Stu>() {
    
			// Sort by age 
			@Override
			public int compare(Stu o1, Stu o2) {
    
				// TODO Auto-generated method stub
				return o1.getAge() - o2.getAge();
			}
		});
		System.out.println("--- After ordering -------");
		for (Stu stu : s) {
    
			System.out.println(stu);
		}
	}
}

class Stu {
    
	private String name;
	private int age;
	public String getName() {
    
		return name;
	}
	public void setName(String name) {
    
		this.name = name;
	}
	public int getAge() {
    
		return age;
	}
	public void setAge(int age) {
    
		this.age = age;
	}
	public Stu(String name, int age) {
    
		super();
		this.name = name;
		this.age = age;
	}
	@Override
	public String toString() {
    
		return "Stu [name=" + name + ", age=" + age + "]";
	}
}

Stu [name= Zhang San , age=19]
Stu [name= Li Si , age=20]
Stu [name= Wang Wu , age=17]
--- After ordering -------
Stu [name= Wang Wu , age=17]
Stu [name= Zhang San , age=19]
Stu [name= Li Si , age=20]

• Array element implementation Comparable Interface

package  aggregate ;

import java.util.Arrays;
import java.util.Comparator;

public class Test {
    
	
	public static void main(String[] args) {
    
		Stu s1 = new Stu(" Zhang San ", 19);
		Stu s2 = new Stu(" Li Si ", 20);
		Stu s3 = new Stu(" Wang Wu ", 17);
		Stu[] s = new Stu[3];
		s[0] = s1;
		s[1] = s2;
		s[2] = s3;
		for (Stu stu : s) {
    
			System.out.println(stu);
		}
		Arrays.sort(s);
		System.out.println("--- After ordering -------");
		for (Stu stu : s) {
    
			System.out.println(stu);
		}
		
	}
}

class Stu implements Comparable<Stu>{
    
	private String name;
	private int age;
	public String getName() {
    
		return name;
	}
	public void setName(String name) {
    
		this.name = name;
	}
	public int getAge() {
    
		return age;
	}
	public void setAge(int age) {
    
		this.age = age;
	}
	public Stu(String name, int age) {
    
		super();
		this.name = name;
		this.age = age;
	}
	@Override
	public String toString() {
    
		return "Stu [name=" + name + ", age=" + age + "]";
	}
	@Override
	public int compareTo(Stu o) {
    
		return this.getAge() - o.getAge();
	}
}

Stu [name= Zhang San , age=19]
Stu [name= Li Si , age=20]
Stu [name= Wang Wu , age=17]
--- After ordering -------
Stu [name= Wang Wu , age=17]
Stu [name= Zhang San , age=19]
Stu [name= Li Si , age=20]