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JUnit is a regression testing framework written by Erich Gamma and Kent Beck. It is used by the developer who implements unit tests in Java.
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package org.junit;

/**
 * Thrown when an {@link org.junit.Assert#assertEquals(Object, Object) assertEquals(String, String)} fails. Create and throw
 * a ComparisonFailure manually if you want to show users the difference between two complex
 * strings.
 *
 * Inspired by a patch from Alex Chaffee ([email protected])
 *
 * @since 4.0
 */
public class ComparisonFailure extends AssertionError {
    /**
     * The maximum length for fExpected and fActual. If it is exceeded, the strings should be shortened.
     *
     * @see ComparisonCompactor
     */
    private static final int MAX_CONTEXT_LENGTH = 20;
    private static final long serialVersionUID = 1L;

    private String fExpected;
    private String fActual;

    /**
     * Constructs a comparison failure.
     *
     * @param message the identifying message or null
     * @param expected the expected string value
     * @param actual the actual string value
     */
    public ComparisonFailure(String message, String expected, String actual) {
        super(message);
        fExpected = expected;
        fActual = actual;
    }

    /**
     * Returns "..." in place of common prefix and "..." in
     * place of common suffix between expected and actual.
     *
     * @see Throwable#getMessage()
     */
    @Override
    public String getMessage() {
        return new ComparisonCompactor(MAX_CONTEXT_LENGTH, fExpected, fActual).compact(super.getMessage());
    }

    /**
     * Returns the actual string value
     *
     * @return the actual string value
     */
    public String getActual() {
        return fActual;
    }

    /**
     * Returns the expected string value
     *
     * @return the expected string value
     */
    public String getExpected() {
        return fExpected;
    }

    private static class ComparisonCompactor {
        private static final String ELLIPSIS = "...";
        private static final String DELTA_END = "]";
        private static final String DELTA_START = "[";

        /**
         * The maximum length for expected and actual. When contextLength
         * is exceeded, the Strings are shortened
         */
        private int fContextLength;
        private String fExpected;
        private String fActual;
        private int fPrefix;
        private int fSuffix;

        /**
         * @param contextLength the maximum length for expected and actual. When contextLength
         * is exceeded, the Strings are shortened
         * @param expected the expected string value
         * @param actual the actual string value
         */
        public ComparisonCompactor(int contextLength, String expected, String actual) {
            fContextLength = contextLength;
            fExpected = expected;
            fActual = actual;
        }

        private String compact(String message) {
            if (fExpected == null || fActual == null || areStringsEqual()) {
                return Assert.format(message, fExpected, fActual);
            }

            findCommonPrefix();
            findCommonSuffix();
            String expected = compactString(fExpected);
            String actual = compactString(fActual);
            return Assert.format(message, expected, actual);
        }

        private String compactString(String source) {
            String result = DELTA_START + source.substring(fPrefix, source.length() - fSuffix + 1) + DELTA_END;
            if (fPrefix > 0) {
                result = computeCommonPrefix() + result;
            }
            if (fSuffix > 0) {
                result = result + computeCommonSuffix();
            }
            return result;
        }

        private void findCommonPrefix() {
            fPrefix = 0;
            int end = Math.min(fExpected.length(), fActual.length());
            for (; fPrefix < end; fPrefix++) {
                if (fExpected.charAt(fPrefix) != fActual.charAt(fPrefix)) {
                    break;
                }
            }
        }

        private void findCommonSuffix() {
            int expectedSuffix = fExpected.length() - 1;
            int actualSuffix = fActual.length() - 1;
            for (; actualSuffix >= fPrefix && expectedSuffix >= fPrefix; actualSuffix--, expectedSuffix--) {
                if (fExpected.charAt(expectedSuffix) != fActual.charAt(actualSuffix)) {
                    break;
                }
            }
            fSuffix = fExpected.length() - expectedSuffix;
        }

        private String computeCommonPrefix() {
            return (fPrefix > fContextLength ? ELLIPSIS : "") + fExpected.substring(Math.max(0, fPrefix - fContextLength), fPrefix);
        }

        private String computeCommonSuffix() {
            int end = Math.min(fExpected.length() - fSuffix + 1 + fContextLength, fExpected.length());
            return fExpected.substring(fExpected.length() - fSuffix + 1, end) + (fExpected.length() - fSuffix + 1 < fExpected.length() - fContextLength ? ELLIPSIS : "");
        }

        private boolean areStringsEqual() {
            return fExpected.equals(fActual);
        }
    }
}