How to define and use Java arrays

1. Basic usage of arrays

1. What is an array

Array: A collection that stores a set of data of the same data type .

2. Define the array

int[]: int type array

double[]: double type array

Variables can be defined by type, such as :

int[] array, array is a variable of this type. This variable is a variable that stores a set of the same data.

Three ways to define arrays:

The first one:

int[] array = {1,2 ,3,4,5,6};Define an array and initialize it

Although new is not written, it is actually an object

Notes:

int[10] array = {1,2,3,4,5,6};Wrong writing, int[] belongs to type, no numbers can be added inside the square brackets, which is equivalent to destruction here its type.

Second type:

int[] array2 = new int[3];

Define array Uninitialized

Third type:

int[] array3 = new int[]{1,2,3,4,5,6};

Definition and initialization

The most commonly used of the three is the first one

3. Use of arrays

Get Array length:

In Java, you can directly find the length of the current array through the array name array.length

Access array elements:

Access the element with index 4 in the array

Access the array element out of bounds:

The Java language directly reports an error when out of bounds

Change array elements:

By placing brackets in the array name, you can not only access the content of the subscript, but also write some data into the subscript

Print array:

The first type: (for loop)

## The second type: (enhanced for loop, for each loop)

The difference between for loop and for each loop:

For loop can get the subscript,

for each loop cannot To the subscript, more are used in collections

The third type: with the help of Java's array manipulation tool class Arrays

2. Arrays as methods Parameters

Basic usage

JVM brief introduction

Storage of local variables in memory :

The reference does not point to an object. Writing method:

This reference does not point to any object

Can a reference point to multiple objects at the same time?

For this code, it can only point to one object and store the address of an object. In the end, only the address of the last object is saved.

The process of passing the array as a parameter of the method:

Solution and printing results:

before Two solutions:

fun2 Print result:

##Analysis example: What does the picture below represent

represents the reference of array2, pointing to the object pointed to by the reference of array1.

The following picture represents the meaning of the above example:

Note:

The quote points to the quote. This sentence is wrong. References can only point to objects

Does the reference have to be on the stack?

Not necessarily. Whether a variable is on the stack is determined by the nature of your variable. If it is a local variable, it must be on the stack. If not, for example, instance member variables are not necessarily on the stack.

3. Array exercises

1. Exchange the values ​​​​of two variables

public class TestDemo {
    public static void swap(int[] array){
        int tmp = array[0];
        array[0] = array[1];
        array[1] = tmp;

    }
    public static void main(String[] args) {
        int[] array = {10,20};
        System.out.println("交换前: "+array[0]+" "+array[1]);
        swap(array);
        System.out.println("交换后: "+array[0]+" "+array[1]);
    }

Print results:

2. Write a method to * 2

/**
    * 在原来的数组上扩大2倍
    * @param array
    */
   public static void enlarge(int[] array){
       for (int i = 0; i <array.length ; i++) {
           array[i] = array[i]*2;
       }

   }

   public static void main(String[] args) {
       int[] array = {1,2,3,4,5,6,7};
       enlarge(array);
       System.out.println(Arrays.toString(array));
   }

Print the result of each element in the array:

Enlarge the original array by twice the value In a new array

/**
     * 把原来数组扩大2倍的值放在一个新的数组中
     * @param array
     * @return
     */
    public static int[] func(int[] array) {
        int[] ret = new int[array.length];
        for (int i = 0; i < array.length; i++) {
            ret[i] = array[i] * 2;
        }
        return ret;
    }

    public static void main(String[] args) {
        int[] array = {1,2,3,4,5,6,7};
       int[] ret =  func(array);
        System.out.println(Arrays.toString(ret));
    }

3. Simulate the implementation of the tostring function

public static String myToString(int[] array){
        String str = "[";

        for (int i = 0; i <array.length ; i++) {
            str = str+array[i];
            if(i != array.length-1){
                str+= ",";
            }
        }
        str= str + "]";
        return str;
    }

    public static void main(String[] args) {
        int[] array = {1,2,3,4,5,6,7};
       String str =  myToString(array);
        System.out.println(str);
    }

Print the result:

4. Find the Maximum element

public static int maxNum(int[] array){
        if(array == null) return -1;
        if (array.length == 0) return -1;
        int max = array[0];
        for (int i = 1; i <array.length ; i++) {
            if(max < array[i]){
                max = array[i];

            }
        }
        return max;
    }

    public static void main(String[] args) {
        int[] array = {12,8,14,26,5,7,8};
        int max = maxNum(array);
        System.out.println(max);
    }

Print result:

5. Find the specified element in the array (sequential search)

public static int findNum(int[] array,int key){
     for (int i = 0; i <array.length ; i++) {
         if(array[i] == key){
             return i;
         }
     }
     return -1;
 }

 public static void main(String[] args) {
     int[] array = {2,4,5,6,11,7,8,9};
     System.out.println(findNum(array, 7));

 }

Print result:

6. Find the specified element in the array (binary search)

二分查找的必要条件是必须有序的数列
    public static int binarySearch(int[] array,int key){
        int left = 0;
        int right = array.length-1;
        while(left <= right){
            int mid = (left+right)/2;
            if(array[mid] > key){
                right = mid - 1;
            }else if(array[mid] < key){
                left = left + 1;
            }else{
                return mid;
            }

        }
        return -1;
    }

    public static void main(String[] args) {
        int[] array = {12,14,15,16,18,23};
        System.out.println(binarySearch(array, 15));
    }

Print result:

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