Java control flow large value array

Directory 1. Control Flow 1.1 Block Scope 1.2 Condition Statement 2. Large Numeric Value 3. Array 3.1 Command Line Parameters

1. Control flow

1.1 Block scope

"Error-prone point": You cannot declare a variable with the same name in two nested blocks, such as

public static void main(String[] args){
    int n;
    ...
    {
        int n; // Error--can't redefine n in inner block
    }
}

This code cannot be compiled because variables with the same name appear in two nested blocks

Note:

In C + +, you can redefine 1 variable in a nested block. Variables defined in the inner layer override variables defined in the outer layer.

1.2 Conditional Statements

"Error-prone point": In a loop, checking whether two floating-point numbers are equal requires special attention, such as

for(double x=0; x!=10; x+=0.1);

The above loop will not end, because 0.1 cannot be accurately expressed in binary, so x will skip from 9.9999999999998 to 10.099999999999998.

"Additional points": switch Statement in the case If break is not added at the end of a branch statement, it is actually a more dangerous situation. So use switch Statement can be written as follows when compiling code javac -Xlint:fallthrough File name. java. If break is missing in a certain bit of a branch, the compiler will report a warning. If you just need some branches and don't add them behind them break You can add 1 annotation to its peripheral method @SuppressWarnings ("fallthrough") so that no warning is generated for the method.

"Additional points": Java No goto Statement, but break It can be labeled, which can be used to realize the purpose of jumping out of the inner loop. Such as

int n;
read_data:
while(){
    for(){
        ...
        break read_data;
        ...
    }
}

Note:

This mode can only jump out of the statement block, but not into the statement block

2. Large values

"Supplementary points": If the basic integers and floating-point numbers do not meet the requirements, you can use the java.math Two classes in the package: case 0 And BigDecinmal . These two classes can handle numeric values containing sequences of numbers of arbitrary length. case 0 Class implements integer operations with arbitrary precision, BigDecinmal The floating-point number operation with arbitrary precision is realized

3. Arrays

"Error-prone point": The array length in Java is not required to be a constant, such as new int[n] An array of length n is created. However, after determining the value of n, it cannot be changed.

3.1 Command Line Parameters

"Supplementary point": Every Java application has a String[] args The main method of the parameter. This parameter indicates that main Method will accept an array of 1 strings, that is, command line arguments. Such as

public class Message{
    public static void main(String[] args){
        if(args.length==0||args[0].equals("-h")){
            System.out.print("Hello,");
        }else if(args[0].equlas("-g")){
            System.out.print("Goodbye,");
        }
        for(int i=1;i<args.length;i++){
            System.out.print(" "+args[i]);
        }
    }
}

If we run this program on the command line using the following form: java Message -g cruel world The following information is displayed Goodbye , cruel world !

This shows that switch 0 The contents of the array memory are as follows: switch 1

Note:

In the main method of the Java application, the program name is not stored in the args array