Basic knowledge of Java

1. java

   • static final int a=0xff;;
   • static final int a=067;
   • static final int a=126;
   • Decimal form 12.37F,-0.423D
   • boolean length 1 byte
   • Basic data type, reference data type
   • toString() converts objects to character
   • Conversion between String type and basic type

2. java+java filename, javac command compiles java source file into class bytecode file.
   java Hellow Word
   javac Hellow Word.java

3. java array: array index 0-A.length-1
   Create array A=new dataType[10]
   Create an array and declare dataType[] A=new dataType[size]

4. Character string

//Compare the difference between the ASCII codes of the first letters in two strings
st1.compareTo(st2);
//Ignore case of letters in characters
st1.compareToIgnoreCase(st2);
st1.compareTo(object.toString());
//Find the position of a character in a string
String s="Hellow World I am lidong";
int last=s.lastIndexOf("lidong");
//Delete a character from a string
String str="Hellow Word!";
str.substring(0,pos)+ str.substring(pos+1);
 //Substitution of letters in strings
 System.out.println(s.replace('l','a'));
 //Character reversal
 String reverse=new StringBuffer(s).reverse().toString();
 System.out.println(reverse);
//Lookup location
int index=s.indexOf("w");
System.out.println(index);
//String splitting
temp=str.split("//");

1. array

  //Array sorting
 System.out.println(array.length);
 Arrays.sort(array,0,9);
 for(int i=0;i<array.length;i++){
 	 System.out.println(array[i]);
  }
//Find the location of the element
int index=Arrays.binarySearch(array,3);
System.out.println("element i In the first place"+index+"Position!");
//Array length
String[][] A=new String[5][2];
//Length of one-dimensional array
System.out.println(A.length);
//Length of 2D array
System.out.println(A[0].length);
//Array inversion
Collections.reverse(arrayList);
//Iterate print traversal array
for(Iterator<String> iterator=arraylist.iterator();iterator.hasNext();System.out.println(iterator.next())){

}
//Get the maximum and minimum values of the array
int min= (int) Collections.min(Arrays.asList(array));
int max=(int) Collections.max(Arrays.asList(array));

1. java time format processing

 Date date=new Date();
 //Define time format
 String dateFormat="yyyy-MM-dd HH:mm:ss";
 SimpleDateFormat s=new SimpleDateFormat(dateFormat);
 System.out.println(s.format(date));

1. Definition of generics

public static <E> void show(E[] array){}

1. For and ForEach recycling

for (int i = 1; i <= 5; i++) {
            for (int j = 5; i <= j; j--) {
                System.out.print(" ");
            }
            for (int j = 1; j <= i; j++) {
                System.out.print("*");
            }
            for (int j = 1; j < i; j++) {
                System.out.print("*");
            }
            System.out.println();
  }
for (int i = 1; i <= 9; i++) {
            for (int j = 1; j <= i; j++) {
                System.out.print(i + "*" + j + "=" + i * j+"\t");
            }
            System.out.println();
        }

1. Exercise topic

//Randomly generate n=10000 floating-point numbers, first output, then sort, then print, and test the sorting time
        int size=100;
        Float[] A=new Float[size];
        for(int i=0;i<A.length;i++){
            A[i]=(float)Math.random()*100;
            System.out.println("The number generated is:");
            System.out.println(A[i]);
        }
        Date d=new Date();
        Long a=System.currentTimeMillis();
        System.out.println("Time before sorting is"+a);
        Arrays.sort(A);
        System.out.println("============================");
        System.out.println("The sorted number is:");
        for(Float element:A){
            System.out.println(element);
        }
        Long b=System.currentTimeMillis();
        System.out.println("The time after sorting is"+b);
        System.out.println("The time difference is"+(b-a));

1. Use of collections

//HashMap traversal, the elements in the collection cannot be duplicate
HashMap<String,String> hm=new HashMap<>();
hm.put("one","Hellow Word!");
hm.put("two","Lidong");
hm.put("three","zhnagsan");
hm.put("one","Hellow!");
System.out.println(hm.get("one"));
System.out.println(hm.get("two"));
System.out.println(hm.get("three"));

//The elements in the HashSet cannot be duplicate. The elements after the HashSet will overwrite the elements before it.
HashSet<String> set = new HashSet<>();
set.add("Zhang San");
set.add("Li Si");
//Duplicate data add failed
set.add("Zhang San");
//for loop traversal set
for (String element : set) {
    System.out.println(element);
}
System.out.println(set.size());
//Traversal with iterator
for(Iterator<String> iterator=set.iterator();iterator.hasNext(); System.out.println(iterator.next())){}

//Traversing the list collection: the elements in the list collection can be duplicate
List<String> list=new ArrayList<>();
list.add("English?");
list.add("Mathematics");
list.add("English?");
//for loop traversal
for(String element:list){
    System.out.println(element);
}
//Iterative traversal
Iterator<String> iterator=list.iterator();
while(iterator.hasNext()){
    System.out.println(iterator.next());
}

//Traverse the key value of HashTable
Hashtable h=new Hashtable();
h.put("one","hello");
h.put("two","hello");
h.put("one","hello");
Enumeration e=h.keys();
while(e.hasMoreElements()){
    System.out.println(e.nextElement());
}

1. java abstract class
   Abstract classes cannot be instantiated. If there are abstract methods in a class, the class must be an abstract class. Abstract methods are not necessarily included in an abstract class.
2. Java notes

  /**
    * javadoc
    * */
	
 //This is the note
 
 /*This is a way*/

1. final: the variable is a constant and can only be assigned once