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How can I check if a single character appears in a string?


Question

In Java is there a way to check the condition:

"Does this single character appear at all in string x"

without using a loop?

2012/05/07
1
214
5/7/2012 11:17:37 AM

Accepted Answer

You can use string.indexOf('a').

If the char a is present in string :

it returns the the index of the first occurrence of the character in the character sequence represented by this object, or -1 if the character does not occur.

2020/03/22
280
3/22/2020 12:40:51 PM


I'm not sure what the original poster is asking exactly. Since indexOf(...) and contains(...) both probably use loops internally, perhaps he's looking to see if this is possible at all without a loop? I can think of two ways off hand, one would of course be recurrsion:

public boolean containsChar(String s, char search) {
    if (s.length() == 0)
        return false;
    else
        return s.charAt(0) == search || containsChar(s.substring(1), search);
}

The other is far less elegant, but completeness...:

/**
 * Works for strings of up to 5 characters
 */
public boolean containsChar(String s, char search) {
    if (s.length() > 5) throw IllegalArgumentException();

    try {
        if (s.charAt(0) == search) return true;
        if (s.charAt(1) == search) return true;
        if (s.charAt(2) == search) return true;
        if (s.charAt(3) == search) return true;
        if (s.charAt(4) == search) return true;
    } catch (IndexOutOfBoundsException e) {
        // this should never happen...
        return false;
    }
    return false;
}

The number of lines grow as you need to support longer and longer strings of course. But there are no loops/recurrsions at all. You can even remove the length check if you're concerned that that length() uses a loop.

2009/02/03

String temp = "abcdefghi";
if(temp.indexOf("b")!=-1)
{
   System.out.println("there is 'b' in temp string");
}
else
{
   System.out.println("there is no 'b' in temp string");
}
2011/07/11

You can use 2 methods from the String class.

  • String.contains() which checks if the string contains a specified sequence of char values
  • String.indexOf() which returns the index within the string of the first occurence of the specified character or substring or returns -1 if the character is not found (there are 4 variations of this method)

Method 1:

String myString = "foobar";
if (myString.contains("x") {
    // Do something.
}

Method 2:

String myString = "foobar";
if (myString.indexOf("x") >= 0 {
    // Do something.
}

Links by: Zach Scrivena

2018/03/27

To check if something does not exist in a string, you at least need to look at each character in a string. So even if you don't explicitly use a loop, it'll have the same efficiency. That being said, you can try using str.contains(""+char).

2009/02/03

If you need to check the same string often you can calculate the character occurrences up-front. This is an implementation that uses a bit array contained into a long array:

public class FastCharacterInStringChecker implements Serializable {
private static final long serialVersionUID = 1L;

private final long[] l = new long[1024]; // 65536 / 64 = 1024

public FastCharacterInStringChecker(final String string) {
    for (final char c: string.toCharArray()) {
        final int index = c >> 6;
        final int value = c - (index << 6);
        l[index] |= 1L << value;
    }
}

public boolean contains(final char c) {
    final int index = c >> 6; // c / 64
    final int value = c - (index << 6); // c - (index * 64)
    return (l[index] & (1L << value)) != 0;
}}
2012/05/07