00|Common mistakes or unclear in Java

00. Multi-variable declaration and initialization

• Simultaneously declare multiple variables of the same type

String a = "Hello", c = "hello";
int x = 5, y = 5;

01. Variable type

01.0 Floating point types

• The default is double type, if you need to specify float type, you canfloat f = 1.0F;

01.1 Type conversion

• If you convert a large type to a small type, you can use coercion, but you will lose precision.

01.2 Reference types

• Note: Classes, Strings, interfaces, and arrays are all reference types.

02. Operators

02.0 Notes on division operations

• 1/2 = 0Analysis: Divisions are displayed by largest type. Here both numbers are of type Int, so the decimal will be discarded
• Division is not rounded.
• 1.0/2 = 0.5or1/2.0 = 0.5

02.1 Notes on assignment operators

• With the assignment operator, the data type does not change
• eg: byte b1=10; b1 = b1+10；This will make the mistake of converting a large type to a small type
• eg: b1 += 10;

02.2 Notes on short-circuit operators

• It will be judged according to the result of the first conditional expression whether to execute the second conditional expression
• Verification program: (if j becomes 21, it means that the subsequent judgment has been executed)

int i=10,j=20;
boolean result = (i>10) && (++j>30);
System.out.println(result);
System.out.println(j);

03 Abstract method && abstract class

• Abstract method: a method that is only declared and has no implementation
  • abstract 返回值类型 方法名（参数）
• Abstract classes: incomplete classes
  • abstract class 类名
  • Abstract classes cannot directly construct objects, that is, Person p = new Person()this is not possible for abstract classes. Need to inherit first, in the construction:

class Chinese extends Person{
    
    ……}
Chinese p = new Person();

• As long as the class contains abstract methods, the class is an abstract class (to add abstract)abstract class Person{……}
• A class is an abstract class, and its methods are not necessarily abstract methods.
• The final modified method cannot be overridden, so abstract and final are not modified at the same time

03.0 Example

abstract class Person{
    
    
	public abstract void eat();
}
class Chinese extends Person{
    
    
	public void eat(){
    
    
		Sysout.out.println("使用筷子吃饭")
	}
}

Chinese p = new Chinese();
p.eat();

04. Interface

• grammar:interface 接口名称{规则属性，规则的行为}
• Interfaces are abstract
• Properties are static, behavior is abstract
• Interface and class are two levels of things
  • Interfaces can inherit from other interfaces
  • The object of the class needs to follow the interface, in java, it is called the implementation (implement). That is: classes need to implement (multiple) interfaces.

04.0 case

	interface USBInterface{
    
    
	
	}
    // 下面的规则符合USBInterface，行为就是powerXXX();
    // 接口中的行为powerXXX是抽象的
	interface USBSupply extends USBInterface{
    
    
		public void powerSupply();
	}
    interface USBReceive extends USBInterface{
    
    
        public void powerReceive();
    }
    // 电脑类中有两个USB接口，接口的功能是提供电源
    class Computer implements USBSupply{
    
    
        public USBReceive usb1;
        public USBReceive usb2; 
        public void powerSupply(){
    
    
            System.out.println("电脑提供能源");
            usb1.powerReceive();
            usb2.powerReceive();
        }
    }
    // 灯
    class Light implements USBReceive{
    
    
        public void powerReceive(){
    
    
            System.out.println("电灯接受能源");
        }
    }
    // main中实现
    Computer c = new Computer();
    Light light = new Light();
    c.usb1 = light;
    c.powerSupply();

05. Bean specification

• Class requirements must have a no-argument, public constructor
• Attributes must be privatized, and getXXX() and setXXX() methods are provided

06. Array of objects

• Declaration method: type [] variable is:String[] names
• Array creation: new type [capacity] ie:new String[3]

String[] names = new String[3];
// 或者声明时，直接赋值
String[] nums = {
    
    "1","3","5"};

07. String class

• Originated from java.lang.String, automatically loaded by the system

String name = "这是字符串";

07.0 Comparison

• Exact equality (including case)a.equals(b)
• equal (regardless of case)a.equalsIgnoreCase(b)
• Comparison of size:a.compareTo(b)

07.1 Interception (substring)

String s = "01234  6789";
s.substring(0,3); //截取0-2，不包含3
s.substring(6);//只传一个参数，表示截取到末尾。6-9

07.2 Split

String s = "01234 6789";
String[] s1 = s.split(" ");//按空格来分割字符串
System.out.println(s1.length);
for(String s2:s1){
    
    
	System.out.println(s2);
}

07.3 Remove spaces (trim)

• This is to remove the leading and trailing spaces of the string

String s = " H e l l o ,mygoodfriend ";
System.out.println(s.trim());
System.out.println("!"+s.trim()+"!");

07.4 Replace (replace, replaceAll)

• Replace all: replace

String s = "Hello World, World Li";
System.out.println(s.replace("World","Java");

• Replace according to your own rules: replaceAll
  If I want to replace both World and Li with Java, I will do the following:

String s = "Hello World, World Li";
System.out.println(s.replaceAll("World|Li","Java");

07.5 Case conversion (toLowerCase, toUpperCase)

String s = "Hello mygoodfriend";
System.out.println(s.toLowerCase());
System.out.println(s.toUpperCase());

If you only want to capitalize the first u of username, how to do it:

String name = "username";
String s1 = name.substring(0,1);
String s2 = name.substring(1);
System.out.println(s1.toUpperCase() + s2);

07.6 Find (charAt, indexOf)

• charAt: Find the character at the specified position by index

String s = "Hello, my friends";
System.out.println(s.charAt(0));

• indexOf: Find the subscript of the first occurrence of the string

String s = "Hello, my friends";
System.out.println(s.indexOf("my"));

• lastIndexOf: Get the subscript of the last occurrence of the string

String s = "Hello, my friends";
System.out.println(s.lastIndexOf("friends"));

07.7 Judgment contains (contains())

• contains: Determines whether the specified string is contained, and returns a Boolean type

String s = "Hello, my friends";
System.out.println(s.contains("mygood"));
System.out.println(s.contains("friends"));

07.8 Judge empty (isEmpty())

String s = "Hello";
System.out.println(s.isEmpty());
String str = "";
System.out.println(str.isEmpty());

08. StringStringBuilder

• This StringBuilder is to optimize the utilization rate caused by continuous creation of new space during splicing operations.
• StringBuilder builds a string, which is a class that provides a large number of string operations.

StringBuilder s = new StringBuilder();
for(int i=0;i<100;++i){
    
    
	s.append(i);	
}
System.out.println(s.toString());

• The efficiency of the above code is higher than the efficiency of the following

String s = "";
for(int i=0;i<100;++i){
    
    
	s = s+i;	
}
System.out.println(s);

08.0 Add (append())

StringBuilder s = new StringBuilder();
s.append("abc");

08.1 Convert to String (toString())

StringBuilder s = new StringBuilder();
s.append("abc");
System.out.println(s.toString());

08.1 Length (length())

StringBuilder s = new StringBuilder();
s.append("abc");
System.out.println(s.length());

08.2 Reverse (reverse())

StringBuilder s = new StringBuilder();
s.append("abc");
System.out.println(s.reverse());

08.3 Insert (insert())

StringBuilder s = new StringBuilder();
s.append("012");
System.out.println(s.insert(1,"dr")); // 在下标为1的地方插入字符串dr