Initialization method of java variables

1. After declaring a variable, the variable must be explicitly initialized using an assignment statement.

2. The variable declaration should be as close as possible to the place where the variable is first used.

3. For local variables, if the type can be inferred from the variable's initial value, there is no need to declare the type.

You only need to use the keyword var without specifying the type.

Example

//Java 尽量保证所有变量在使用前都能得到恰当的初始化，否则会报异常
//Java中提供了四种初始化方法：默认初始化、显式赋值初始化、调用类对象初始化和调用方法初始化。
 
//错误写法
void f() {
    int i;
    i++;
}
//执行时会报异常，原因是变量i没有初始化
 
 
 
## 成员变量初始化方式之一————默认初始化：
 
public class InitialValues {
    boolean t;
    char c;
    byte b;
    short s;
    int i;
    long l;
    float f;
    double d;
    InitialValues reference;
 
    void printInitialValues() {
        System.out.println("Data type Initial value");
        System.out.println("boolean " + t);
        System.out.println("char[" + c + "]");
        System.out.println("byte " + b);
        System.out.println("short " + s);
        System.out.println("int " + i);
        System.out.println("long " + l);
        System.out.println("float " + f);
        System.out.println("double " + d);
        System.out.println("reference " + reference);
    }
 
    public static void main(String[] args) {
        new InitialValues().printInitialValues();
    }
}
输出：
Data type Initial value
boolean false  
char[]   //char 的默认值为 0或者写为'\u0000'，所以显示为空白
byte 0
short 0
int 0
long 0   //0L
float 0.0  //0.0F
double 0.0  
reference null
 
## 成员变量初始化方式之二————显式赋值：
public class InitialValues2 {
 boolean bool = true;
 char ch = 'x';
 byte b = 47;
 short s = 0xff;
 int i = 999;
 long lng = 1;
 float f = 3.14f;
 double d = 3.14159;
}
 
## 成员变量初始化方式之三————调用对象进行初始化赋值：
class Depth {
}
public class Measurement {
    Depth d = new Depth();
    // ...
}
//注意：如果没有为 d 赋予初值就尝试使用它，就会出现运行时错误，会产生异常。
 
## 成员变量初始化方式之四————调用方法进行初始化赋值：
//1、调用无参方法
public class MethodInit {
    int i = f();
    
    int f() {
        return 11;
    }
    
}
 
//2、调用有参方法
public class MethodInit2 {
    int i = f();
    int j = g(i);
    
    int f() {
        return 11;
    }
    
    int g(int n) {
        return n * 10;
    }
}
//注意：调用有参方法时，方法中的参数不能是未初始化的类成员变量，否则会报异常

The above is the detailed content of Initialization method of java variables. For more information, please follow other related articles on the PHP Chinese website!