java: How to check StringBuilder charcters to see if it contains same characters as new string request from array?

I want to check if a Java StringBuilder object contains a character.

For 开发者_开发知识库example I have a list of foods in an array {apple, pear, orange, peach, cherry} and I want to add a random number of these food types to the StringBuilder.

How would I check if StringBuilder contains a character?

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The StringBuilder class does not have a contains() method, but it does have an indexOf() method. The indexOf() method returns -1 when the substring was not found.

Example:

stringBuilder.indexOf(charactersYouWantToCheck)!=-1

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The StringBuilder class does not have a contains() method, but String does. So you can use StringBuilder to build a string by using toString(). Then call contains() on it, to check if it contains that character or string.

Example:

stringBuilder.toString().contains(characterYouWantToCheck)

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It is of course possible to check the string by iterating through it (e.g. use indexOf()), but I would use a Set in this particular instance to add the fruits (if it already exists it will not be added again), once you're done adding fruits convert the Set into a String, e.g.

Set<String> fruits = new HashSet<String>();
for (String fruit: fruitSource) {
    fruits.add(fruit);
}
StringBuilder sb = new StringBuilder();
for (String fruit: fruits) {
    sb.append(fruit);
    sb.append(", ");
}
return sb.toString();

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A different option is to create a List<String> of all of the options, and use Collections.shuffle to randomly permute the list. You can then join together the first n elements.

Since both Collections.shuffle and joining together the first n elements takes linear time, the entire process should take linear time.

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You shouldn't be "checking" anything to do that, you should do it so that duplicates never get added in the first place. To do that:

1. start with a List<String> of the options
   • copy the list
   • add a random element to the result
   • remove that element.
   • Go back to 3 until you've added as many elements as you want or there aren't anymore left

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import java.util.Random;

public class Test {
    public static void main(String args[]) {
        String[] fruits = { "apple", "pear", "orange", "peach", "cherry" };

        StringBuilder sb = new StringBuilder("Some fruits: apple");

        Random r = new Random();
        int N = 3;
        for (int i = 0; i < N; i++) {
            String toAdd;
            do {
                toAdd = fruits[r.nextInt(fruits.length)];
            } while (sb.indexOf(toAdd) != -1);

            sb.append(", " + toAdd);
        }

        System.out.println(sb);
    }    
}

Output:

Some fruits: apple, pear, orange, peach

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But I agree with Michael Borgwardt on that it's a bad thing to check using index-of over and over again.

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A simple way to do it is similar to Marc van Kempen's solution but smaller.

Set<String> fruits = new HashSet<String>();
StringBuilder sb = new StringBuilder();
for (String fruit: fruits) {
    if(fruits.add(fruit)){
        sb.append(fruit);
        sb.append(", ");
    }
}
return sb.toString();

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strb2.indexOf(String.valueOf(strb1.charAt(i))

strb2 and strb1 are objects of StringBuilder class. If the letter is present, it will return its index else -1.

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Java StringBuilder doesn't have contains method So you can check whether a string or character contains in the StringBuilder as below:

StingBuilder sb = new StringBuilder("apple, pear, orange, peach, cherry");
if(sb.indexOf("someFruit") == -1) {
   //you can insert
} else {
   // do something
}

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//StringBuilder object

    StringBuilder sb = new StringBuilder("One Two Three");
    
    System.out.println( contains(sb, "One") );
    System.out.println( contains(sb, "Two") );
    System.out.println( contains(sb, "Five") );

Output:

true
true
false

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StringBuilder doesn't have a contains() method, but it does have the indexOf() method. Therefore, you can check for the index of character and if it returns -1, the character is not present in the StringBuilder.

StringBuilder sb = new StringBuilder();

if(sb.indexOf(CharacterYouWantToCheck) != -1){
   //sb contains character
}

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Here I iterate through the String to find a matching char in the StringBuilder using chars().forEach()

public static void findMatch(String s){

    StringBuilder sb = new StringBuilder();
    sb.append("can");
    s.chars().forEach(i -> {

        if(sb.indexOf(String.valueOf((char)i))!= -1)
            System.out.println("Found char is "+String.valueOf((char)i));
    });