Erudite Valley Learning Records] Super summary, attentive sharing | common APIs

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Commonly used API

Math Common methods of class

 Method name   explain 
public static int abs(int a)  Returns the absolute value of the parameter 
public static double ceil(double a)  Rounding up 
public static double floor(double a)  Rounding down 
public static int round(float a)  rounding 
public static int max(int a,int b)  Return to two int The larger of the values 
public static int min(int a,int b)  Return to two int The smaller of the values 
public static double pow(double a,double b)  return a Of b The value of the power 
public static double random()  The return value is double Positive value of ,[0.0,1.0)

System Class Overview

System Can't be instantiated

• public static void exit(int status) Terminate the currently running Java virtual machine ,0 It means normal exit , Non zero means abnormal exit

• public static long currentTimeMillis() Return current time ( In Milliseconds )

• arraycopy( Data source array , Starting index , Destination array , Starting index , Number of copies ) Array copy

		// arraycopy( Data source array ,  Starting index ,  Destination array ,  Starting index ,  Number of copies )
        int[] arr1 = {
    1, 2, 3, 4, 5};
        int[] arr2 = new int[10];
        System.arraycopy(arr1, 0,arr2,0,arr1.length);
        for (int i = 0; i < arr2.length; i++) {
    
            System.out.print(arr2[i]); // 1234500000
        }

Object Class overview

Each class can put Object As parent . All classes inherit directly or indirectly from this class
Construction method ：public Object()

• Object Class is the direct or indirect parent of all classes
• To print an object directly is to print the object toString Return value of method
• Object Class toString Method gets the address value of the object
• We are usually right about toString Method
• public String toString() Returns the string representation of the object . It is recommended that all subclasses override this method , Automatic generation
• public boolean equals( Another object ) Compare objects for equality . Default comparison address , Rewriting can compare content , Automatic generation

Objects Common methods of class

• public static String toString( object ) Returns the string representation of the object in the parameter .
• public static String toString( object , Default string ) Returns the string representation of the object .
• public static Boolean isNull( object ) Determines if the object is null
• public static Boolean nonNull( object ) Judge whether the object is not empty

BigDecimal Common methods of class

effect ： Can be used to accurately calculate
public BigDecimal add( the other one BigDecimal object ) Add
public BigDecimal subtract ( the other one BigDecimal object ) Subtraction
public BigDecimal multiply ( the other one BigDecimal object ) Multiplication
public BigDecimal divide ( the other one BigDecimal object ) division
public BigDecimal divide ( the other one BigDecimal object , How many exactly , rounding mode ) division

BigDecimal divide = bd1.divide( The objects involved in the operation , How many places after the decimal point , rounding mode );

		BigDecimal bd1 = new BigDecimal("7.7");
        BigDecimal bd2 = new BigDecimal("3");
        BigDecimal divide = bd1.divide(bd2, 2, BigDecimal.ROUND_UP);//  Jin Yi 
        BigDecimal divide2 = bd1.divide(bd2, 2, BigDecimal.ROUND_FLOOR);//  Decapitation 
        BigDecimal divide3 = bd1.divide(bd2, 2, BigDecimal.ROUND_HALF_UP);//  rounding 
        System.out.println(divide3);//2.57
        System.out.println(divide2);//2.56
        System.out.println(divide);// 2.57

Basic type packaging

Automatic boxing : Put a basic data type The wrapper class corresponding to the variable ,java The bottom layer will help us automatically call valueOf Method
Automatic dismantling : Put a packaging type Become the corresponding basic data type
Integer： A basic data type is wrapped in this object int Value
Method name explain
public Integer(int value) according to int Value creation Integer object ( obsolete )
public Integer(String s) according to String Value creation Integer object ( obsolete )
public static Integer valueOf(int i) Returns the specified int It's worth it Integer example
public static Integer valueOf(String s) Returns a... That holds the specified value Integer object String

Integer i3 = Integer.valueOf(200); // Returns the specified int It's worth it Integer example 
Integer i4 = Integer.valueOf("200"); //  Returns a... That holds the specified value Integer object String
System.out.println(i3); // 200
System.out.println(i4); // 200

Be careful ： When using wrapper class types , If operation , It's best to judge whether it is null
What we recommend is , As long as it's an object , Before use, it must be carried out not for null The judgment of the

1. int Convert to String
   Mode one ： Just add double quotation marks
   Mode two ：public static String valueOf(int i)： return int String representation of parameter . The method is String Methods in class
2. String Convert to int
   public static int parseInt(String s)： Parse string to int type . The method is Integer Methods in class

		String s1 = "100";
        int i2 = Integer.parseInt(s1);
        System.out.println(i2+i1); // 300

Arrays Class overview and common methods

• public static String toString(int[] a) Returns a string representation of the contents of the specified array
• public static void sort(int[] a) Arranges the specified array in numerical order
• public static int binarySearch(int[] a, int key) Use binary search to return the index of the specified element

		int[] arr = {
    3,2,4,6,7};
        System.out.println(Arrays.toString(arr)); // [3, 2, 4, 6, 7]
        Arrays.sort(arr);
        System.out.println(Arrays.toString(arr)); // [2, 3, 4, 6, 7]

        int index = Arrays.binarySearch(arr, 7);
        System.out.println(index);  //4