catalogue

1, Arrays class

summary

Member method

2, Overview of basic types and packaging classes

What is a basic type wrapper class

Packaging classes corresponding to various basic data types

Integer class

3, Conversion between String and int types

int --> String

String --> int

4, Automatic packing and unpacking of JDK5

Case demonstration:

matters needing attention

1, Arrays class

1. summary

   A tool class that operates on arrays.

   It provides functions such as sorting and searching.

2. Member method

   public static String toString(int[] a)  //Converts the passed in integer array into a string and returns

       
   Example:
   
   

       import java.util.Arrays;
       public class Test3 {
           public static void main(String[] args) {
               int[] a ={1,2,3,5,4,8,2,6,52};
               System.out.println(Arrays.toString(a));
           }
       }
       

    public static void sort(int[] a)    //Sort the passed in integer array

      
   Example:
     
   

     import java.util.Arrays;
       public class Test3 {
           public static void main(String[] args) {
               int[] a ={1,2,3,5,4,8,2,6,52};
               Arrays.sort(a);
               System.out.println(Arrays.toString(a));
           }
       }

    public static int binarySearch(int[] a,int key) //Binary search, the array passed in must be ordered

       
      
   Example:
   
   

       import java.util.Arrays;
       public class Test3 {
           public static void main(String[] args) {
               int[] a ={1,2,3,5,4,8,2,6,52};
               Arrays.sort(a);
               System.out.println(Arrays.binarySearch(a, 5));
           }
       }
       

    static boolean equals(int[] a, int[] a2) //Compare the elements in the two arrays. Are they the same

       
      
   Example:
   
   

       import java.util.Arrays;
       public class Test3 {
           public static void main(String[] args) {
               int[] a = {1, 2, 3, 5, 4, 8, 2, 6, 52};
               int[] b = {1, 2, 3, 5, 4, 8, 2, 6, 52};
               int[] c = {1, 2, 3, 5};
               System.out.println(Arrays.equals(a, b));
               System.out.println(Arrays.equals(b, c));
           }
       }

   static int[] copyOf(int[] original, int newLength) 
    //Copy the elements from the old array to a new array,
   //The new array length is newLength. Copy the old array from 0

   Example:
   
   

       import java.util.Arrays;
       public class Test3 {
           public static void main(String[] args) {
               int[] a = {1, 2, 3, 5, 4, 8, 2, 6, 52};
               int[] b = Arrays.copyOf(a, 5);
               System.out.println(Arrays.toString(b));
           }
       }

   static int[] copyOfRange(int[] original, int from, int to) 
   //Copy several elements in the specified range in the old array to the new array from start index to end index

   Example:
   

       import java.util.Arrays;
       public class Test4 {
           public static void main(String[] args) {
               int[] arr = {1, 23, 4, 6, 20};
               int[] arr1 = Arrays.copyOfRange(arr,0,3);
               System.out.println(Arrays.toString(arr1));
           }
       }

2, Overview of basic types and packaging classes

1. What is a basic type wrapper class

   In order to perform more and more convenient operations on basic types, java provides corresponding class types for each basic data type. The basic type wrapper class is to wrap the function methods of basic types such as int, short, byte and char in the corresponding class for our convenience.

2. Packaging classes corresponding to various basic data types

   byte        Byte
   short       Short
   int         Integer
   long        Long
   float       Float
   double      Double
   char        Character
   boolean     Boolean

3. Integer class

   summary:

   The Integer class wraps a value of the basic type int in the object.
   ​
   This class provides multiple methods that can convert between int and String types.
   ​
   There are also some other constants and methods that are very useful for handling int types.

   Construction method:

   //Construct a new Integer object that represents the specified int value
   public Integer(int value)
   //Construct a new Integer object, which represents the int value indicated by the String parameter, and s is the String to be converted to Integer.
   public Integer(String s)

   method:

   //Returns a string representation of an integer parameter as a binary (Radix 2) unsigned integer
   public static String toBinaryString(int i)
   //Returns a string representation of an integer parameter as an octal (Radix 8) unsigned integer
   public static String toOctalString(int i)
   //Returns an unsigned integer as a hexadecimal argument
   public static String toHexString(int i)

3, Conversion between String and int types

1. int --> String

   1. Empty string splicing
    2. valueOf method: public static String valueOf(int i)
   3. Complete through Integer
    4. toString method: public static String toString(int i)

   The example is completed by Integer:

   public class Test6 {
       public static void main(String[] args) {
           Integer integer = new Integer(100);
           System.out.println(integer.toString());
       }
   }

2. String --> int

   1. Complete through Integer
    2. parseInt method: public static int parseInt(String s)

   The example is completed by Integer:

   public class Test6 {
       public static void main(String[] args) {
           Integer integer = new Integer("100");
           System.out.println(integer);
       }
   }

4, Automatic packing and unpacking of JDK5

1. Automatic packing: convert basic type to packing type;

   Automatic unpacking: convert packaging type to basic type.

2. Case demonstration:

   public class Test6 {
       public static void main(String[] args) {
           Integer i =10;  //Automatic packing
           i+=20;  //Automatic unpacking
       }
   }

3. matters needing attention

   When used, Integer x = null; NullPointerException will appear in the code. It is recommended to judge whether it is null before using it.