src/org/junit/ComparisonFailure.java

package org.junit;

/**
 * Thrown when an {@link org.junit.Assert#assertEquals(Object, Object) assertEquals(String, String)} fails. Create and throw
 * a <code>ComparisonFailure</code> manually if you want to show users the difference between two complex 
 * strings.
 * 
 * Inspired by a patch from Alex Chaffee (alex@purpletech.com)
 */
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 <code>expected</code> and <code>actual</code>. When <code>contextLength</code> 
		 * 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 <code>expected</code> and <code>actual</code>. 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);
		}
	}
}