Java Set equality ignore case

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:

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3:

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()