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International Component for Unicode for Java (ICU4J) is a mature, widely used Java library providing Unicode and Globalization support

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// © 2016 and later: Unicode, Inc. and others.
// License & terms of use: http://www.unicode.org/copyright.html
/**
 *******************************************************************************
 * Copyright (C) 2001-2013, International Business Machines Corporation and    *
 * others. All Rights Reserved.                                                *
 *******************************************************************************
 */
package com.ibm.icu.util;

import com.ibm.icu.lang.UCharacter;

/**
 * A string used as a key in java.util.Hashtable and other
 * collections.  It retains case information, but its equals() and
 * hashCode() methods ignore case.
 * @stable ICU 2.0
 */
public class CaseInsensitiveString {

    private String string;

    private int hash = 0;

    private String folded = null;

    private static String foldCase(String foldee)
    {
        return UCharacter.foldCase(foldee, true);
    }

    private void getFolded()
    {
        if (folded == null) {
            folded = foldCase(string);
        }
    }

    /**
     * Constructs an CaseInsentiveString object from the given string
     * @param s The string to construct this object from
     * @stable ICU 2.0
     */
    public CaseInsensitiveString(String s) {
        string = s;
    }
    /**
     * returns the underlying string
     * @return String
     * @stable ICU 2.0
     */
    public String getString() {
        return string;
    }
    /**
     * Compare the object with this
     * @param o Object to compare this object with
     * @stable ICU 2.0
     */
    @Override
    public boolean equals(Object o) {
        if (o == null) {
            return false;
        }
        if (this == o) {
            return true;
        }
        if (o instanceof CaseInsensitiveString) {
            getFolded();
            CaseInsensitiveString cis = (CaseInsensitiveString) o;
            cis.getFolded();
            return folded.equals(cis.folded);
        }
        return false;
    }

    /**
     * Returns the hashCode of this object
     * @return int hashcode
     * @stable ICU 2.0
     */
    @Override
    public int hashCode() {
        getFolded();

        if (hash == 0) {
            hash = folded.hashCode();
        }

        return hash;
    }

    /**
     * Overrides superclass method
     * @return a string representation of the object.
     * @stable ICU 2.0
     */
    @Override
    public String toString() {
        return string;
    }
}