如何使用java进行子字符串和连接

Question

Anyone can guide me? Example get 1234 from the user then I store in a variable then substring and concat by adding "-" 12-34 then store in oracle. It returns an error.

String a;
String b;
      public class concatab{
      public static void main(String[] args) {
            String a1 = a.substring(1,2);
            String b1 = b.substring(3,2);
            System.out.println(ab);
            String c1 = a1 + "-" + b1;
        }
 }

Answer 1

I couldn't quite tell what you were trying to do with your code. However, here's an example that works for reference:

public class Main {
  static String a = "1234";

  public static void main(String[] args) {
    String a1 = a.substring(0, 2);
    String b1 = a.substring(2);
    String c1 = a1 + "-" + b1;
    System.out.println(c1);
  }
}

1. Class members need to be inside a class.
2. Calling the substring method on your strings will throw a NullPointerException as Strings a and b are initialized to null
3. You cannot call substring with the start index being greater than the end index. Doing so will throw an exception
4. String a and b must be declared static, as they are referenced from a static context.

Answer 2

Remember, the compiler reads code line by line.

Your code is a set of instructions for the computer to carry out.

Try to keep that in mind when creating code. You tried to concatenate after you printed. Therefore the computer will print the concatenation before you concatenate your intended string.

And if you're trying to get any sort of user input, you have to prompt the user. You can do that by using the Scanner class. But for the sake of simplicity, let's manually enter the number ourselves.

First thing, your variables aren't declared in any scope. They must be declared, at least within the curly braces of the class.

I'm not sure if you have any other methods, but your variables aren't initialized. In consequence you're trying to find the substring for a string that doesn't exist.

So, let's look at the code and go from there:

public class ConcatAB{
public static void main(String[] args) {
    String a = "0123";
    String b = "123456";
        String a1 = a.substring(1,3);
        String b1 = b.substring(2,4);
        String c1 = a1 + "-" + b1;
        System.out.println(c1);
    }

}

Strings are arrays of characters. And because a string is an array, it's index begins at 0. And the way substrings work, they cut from the index, not the element.

When we get the substring and assign its value to string a1, we cut a from (0,3) as opposed to (1,2). We use 0 and 3 because the cut includes the first number in the parenthesis and cuts the number before the last number in the parentheses. Same goes for string b1.

Now, when concatenating, we don't even need to assign it a variable we could just print it. Like:

System.out.println(a1 + "-" + b1);

But I'm not to sure if you want to save that string for another purpose.

I hope this helps.