Java Set<String> equality ignore case

ぃ、小莉子 提交于 2019-11-27 06:34:55

问题


I want to check if all elements of two sets of String are equal by ignoring the letter's cases.

Set<String> set1 ;
Set<String> set2 ;
.
.
.
if(set1.equals(set2)){ //all elements of set1 are equal to set2 
 //dosomething
}
else{
 //do something else
}

However, this equality check doesn't ignore the cases of the string. Is there some other way of doing that?


回答1:


Alternatively you can use TreeSet.

public static void main(String[] args){
    Set<String> s1 = new TreeSet<String>(String.CASE_INSENSITIVE_ORDER);
    s1.addAll(Arrays.asList(new String[] {"a", "b", "c"}));

    Set<String> s2 = new TreeSet<String>(String.CASE_INSENSITIVE_ORDER);
    s2.addAll(Arrays.asList(new String[] {"A", "B", "C"}));

    System.out.println(s1.equals(s2));
}



回答2:


Unfortunately, Java does not let you supply an external "equality comparer": when you use strings, HashSet uses only built-in hashCode and equals.

You can work around this problem by populating an auxiliary HashSet<String> with strings converted to a specific (i.e. upper or lower) case, and then checking the equality on it, like this:

boolean eq = set1.size() == set2.size();
if (eq) {
    Set<String> aux = new HashSet<String>();
    for (String s : set1) {
        aux.add(s.toUpperCase());
    }
    for (String s : set2) {
        if (!aux.contains(s.toUpperCase())) {
            eq = false;
            break;
        }
    }
}
if (eq) {
    // The sets are equal ignoring the case
}



回答3:


Untested, but this is the general idea:

public boolean setEqualsIgnoreCase(Set<String> a, Set<String>b)
{
    if (a.size() != b.size()) return false;
    Iterator<String> ai = a.iterator();
    Iterator<String> bi = b.iterator();
    while(ai.hasNext())
    {
         if (!ai.next().equalsIgnoreCase(bi.next())) return false;
    }
    return true;
}



回答4:


Not that I know of.

The best solution I can see, albeit over-engineered, would be to create your custom holder class holding a String instance field (String is final and cannot be inherited).

You can then override equals / hashCode wherein for two String properties equalsIgnoreCase across two instances, equals would return trueand hashCodes would be equal.

This implies:

  • hashCode returns a hash code based on a lower (or upper) cased property's hash code.
  • equals is based on equalsIgnoreCase

    class MyString {
        String s;
    
        MyString(String s) {
            this.s = s;
        }
        @Override
        public int hashCode() {
            final int prime = 31;
            int result = 1;
            result = prime * result + ((s == null) ? 0 : s.toLowerCase().hashCode());
            return result;
        }
    
        @Override
        public boolean equals(Object obj) {
            if (this == obj)
                return true;
            if (obj == null)
                return false;
            if (getClass() != obj.getClass())
                return false;
            MyString other = (MyString) obj;
            if (s == null) {
                if (other.s != null)
                    return false;
            }
            else if (!s.equalsIgnoreCase(other.s))
                return false;
            return true;
        }
    
    }
    public static void main(String[] args) {
            Set<MyString> set0 = new HashSet<MyString>(
                Arrays.asList(new MyString[]
                    {
                        new MyString("FOO"), new MyString("BAR")
                    }
                )
            );
            Set<MyString> set1 = new HashSet<MyString>(
                Arrays.asList(new MyString[]
                    {
                        new MyString("foo"), new MyString("bar")
                    }
                )
            );
            System.out.println(set0.equals(set1));
     }
    

Output

true

... as said, over-engineered (but working).




回答5:


I would build something like this (in some form of Java pseudo code):

Set<String> set1;
Set<String> set2;

if (set1.size() != set2.size()) {
  return NOT_EQUAL;
} else {
  Set<String> set3 = new HashSet<String>();
  for (String s: set1) set3.add(s.toUpperCase());
  for (String s: set2) set3.add(s.toUpperCase());
  return set1.size() == set3.size() ? EQUAL : NOT_EQUAL;
}



回答6:


You can use a loop and equalsIgnoreCase

testString.equalsIgnoreCase()


来源:https://stackoverflow.com/questions/24558456/java-setstring-equality-ignore-case

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