if(s1.Contains(s2)) 似乎总是正确的

Question

sorry, changed the question slightly.

essentially i want to know if aString contains String . My issue is when comparing say aS a substring of aString ) "aS".contains("String") shows true.

String a="st", b="string"; I ran System.out.println(a.contains(b)); That returned false , as expected. I have an understanding of contains, i must be missing something else.

So It had seemed that my program was working properly, but I made some adjustments and came back and the whole thing stopped working. I sussed out what are usually the common culprits (brackets, file io, etc.) . I found if(string.contains(string)) would continually run, ie: always true. not sure why this is happening, probably something I missed in the code.

This is an example of my output (Just a char by char reading of the file):

I nteger G ;

import java.io.File;
import java.util.ArrayList;
import java.util.Scanner;

public class comp{

    public static void main(String[] args){
        ArrayList<String> lines = new ArrayList<String>();
        ArrayList<String> symbolTable = new ArrayList<String>();
        ArrayList<String> parsedFile = new ArrayList<String>();


         try {
                File file = new File("symbolTable.txt");
                Scanner scanner=new Scanner(file);
                while (scanner.hasNextLine()&&symbolTable.add(scanner.nextLine().replaceAll("\\s+","").toLowerCase()));
                scanner.close();
            } catch (Exception ex) {
                ex.printStackTrace();
            }


         try {
                File file = new File("APU_CS400_input.txt");
                Scanner scanner=new Scanner(file);
                while (scanner.hasNextLine()&&lines.add(scanner.nextLine().replaceAll("\\s+","").toLowerCase()));
                scanner.close();
            } catch (Exception ex) {
                ex.printStackTrace();
            }


         //runs through line by line of the input file
         for(String line: lines){
             String sBuild = "";
             StringBuilder identifier = new StringBuilder("");

             //moves through the line char by char
             for(int i=0;line.length()>i; i++){
                 sBuild+=line.charAt(i);
                 //moves through the symbol table comparing each symbol to each string 
                 //that is built char by char
                 for(String symbol: symbolTable){
                     //if the char string matches the symbol then any identifiers are saved and 
                     //symbols are saved, the string is then reset to empty
            //This is where i seem to get an issue

        ***if(sBuild.contains(symbol)){***
                        if(symbol.length()<sBuild.length()){
                            identifier.append(sBuild,0,sBuild.length()-symbol.length()); 
                            parsedFile.add(identifier.toString());
                            identifier.delete(0,sBuild.length()-symbol.length());
                        }
                        sBuild="";
                        parsedFile.add(symbol);
                     }

                 }
             }
         }


         for(String symbol:parsedFile){
             System.out.println(symbol);
         }

    }

}

Blockquote 

Answer 1

Think of it this way.

s1.contains(s2)

should return true , if a substring of s1 can be found such that

s1.substring(i, j).equals(s2)

is true.

If s2 is an empty string, then i = 0, j = 0 is one such substring, so contains() returns true .

As it should.

Answer 2

if(String.Contains(""))总是应该为真，只要字符串不为空。

Answer 3

You may have to look at this one a bit mathematically to see why s.contains("") is always true. Suppose you think of this this way:

a.contains(b) is true if there are some values i and j such that a.substring(i,j) is equal to b .

If you think about it a bit, you'll see that this is exactly what contains means when the argument is a nonempty string like "xyz" . If there is some substring of x that equals "xyz" , then s.contains("xyz") is true. If there is no such substring, then s.contains("xyz") is false.

So it makes sense that the same logic would apply for an empty string, since it applies everywhere else. And it's always true that a.substring(0,0) equals "" (if a is not null). That's why a.contains("") should always be true.

It may not be intuitively obvious from the English meaning of "contains", but when you're dealing with "edge cases" like this, you sometimes have to think in different terms. Often, the Javadoc spells things out so that you can easily figure out what happens in the edge cases, without relying on intuition. Unfortunately, in this case, they didn't.

Answer 4

essentially i want to know if "aString" contains "String".

Yes, "aString" as a string-value does contain the string-value of "String"

My issue is when comparing say "aS" (a substring of "aString") "aS".contains("String") shows true.

Are you sure? This cannot be , therefore I rather suspect bugs in your code.

---

To spare youself of "empty String symbols" consider this:

try {
  File file = new File("symbolTable.txt");
  Scanner scanner=new Scanner(file);
  while (scanner.hasNextLine()) {
    // toLowerCase will do nothing for characters that are not letters
    // Don't spend CPU cycles with regex
    String symbolLine=scanner.nextLine().toLowerCase();
    // Collect the symbol only if non-empty?? This will save you from empty symbols
    if(symbolLine.trim().length()>0) {
      symbolTable.add(symbolLine); // or .add(symbolLine.trim()) ???
    }
  }
  scanner.close();
} catch (Exception ex) {
  ex.printStackTrace();
}