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Java SE: Tips

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1. How to compare equals for two List:

    1> Writing boiler plate code

    2> Use org.apache.commons.collections.ListUtils.isEqualList()

         Benefit: Help us handle boiler plate code for null check.

public static boolean isEqualList(final Collection list1, final Collection list2) {
    if (list1 == list2) {
        return true;
    }
    if (list1 == null || list2 == null || list1.size() != list2.size()) {
        return false;
    }

    Iterator it1 = list1.iterator();
    Iterator it2 = list2.iterator();
    Object obj1 = null;
    Object obj2 = null;

    while (it1.hasNext() && it2.hasNext()) {
        obj1 = it1.next();
        obj2 = it2.next();

        if (!(obj1 == null ? obj2 == null : obj1.equals(obj2))) {
            return false;
        }
    }

    return !(it1.hasNext() || it2.hasNext());
}

 

Set Operations:

    1. How to compare equals for two Sets?

        Use org.apache.commons.collections.SetUtils.isEqualSet():

        Benefit: Help us handle boiler plate code for null check.

public static boolean isEqualSet(final Collection set1, final Collection set2) {
    if (set1 == set2) {
        return true;
    }
    if (set1 == null || set2 == null || set1.size() != set2.size()) {
        return false;
    }

    return set1.containsAll(set2);
}

   2. How to operate subtraction between two Sets?

       Use com.google.common.collect.Sets.difference()

@Test
public void subtractTest() {
  Set<String> strSet1 = Sets.newHashSet("A", "B", "C", "D");
  Set<String> strSet2 = Sets.newHashSet("D", "M", "O", "G");

  Set<String> strSet1SubtractStrSet2 = Sets.difference(strSet1, strSet2);
  assertTrue(SetUtils.isEqualSet(Sets.newHashSet("A", "B", "C"),
	strSet1SubtractStrSet2));
  Set<String> strSet2SubtractStrSet1 = Sets.difference(strSet2, strSet1);
	assertTrue(SetUtils.isEqualSet(Sets.newHashSet("M", "O", "G"),
	strSet2SubtractStrSet1));
}

   3. How to operate union between two Sets?

       Use com.google.common.collect.Sets.union()

   4. How to operate intersection between two Sets?

       Use com.google.common.collect.Sets.intersection()

   5. How to operate symmetric difference between two Sets?

       Use com.google.common.collect.Sets.symmetricDifference()

@Test
public void symmetricDifferenceTest() {
  Set<String> strSet1 = Sets.newHashSet("A", "B", "C", "D");
  Set<String> strSet2 = Sets.newHashSet("D", "M", "O", "G");

  Set<String> symmetricDifference = Sets.symmetricDifference(strSet1,
	strSet2);

  assertTrue(SetUtils.isEqualSet(
	Sets.newHashSet("A", "B", "C", "M", "O", "G"),
	symmetricDifference));
}

 

 

String Operation:

    1> Strings.isNullOrEmpty()

    2> Strings.nullToEmpty()

    3> How to swap case for a String?

import static org.junit.Assert.assertEquals;

import org.apache.commons.lang3.StringUtils;
import org.junit.Test;

public class StringTest {
    @Test
    public void swapCaseTest() {
	String str = "Hello";
	assertEquals("hELLO", StringUtils.swapCase(str));
    }
}
        StringUtils.swapCase source code:
public static String swapCase(final String str) {
    if (StringUtils.isEmpty(str)) {
        return str;
    }

    final char[] buffer = str.toCharArray();

    for (int i = 0; i < buffer.length; i++) {
        final char ch = buffer[i];
        if (Character.isUpperCase(ch)) {
            buffer[i] = Character.toLowerCase(ch);
        } else if (Character.isTitleCase(ch)) {
            buffer[i] = Character.toLowerCase(ch);
        } else if (Character.isLowerCase(ch)) {
            buffer[i] = Character.toUpperCase(ch);
        }
    }
    return new String(buffer);
}

    4> How to remove all spaces in a String?

@Test
public void removeSpaceTest() {
  String str = "Hello world, I am  Davy";
  String removedStr = StringUtils.remove(str, ' '); // Find space from target String char by char
  assertEquals("Helloworld,IamDavy", removedStr);

  removedStr = str.replaceAll("\\s", ""); // Find space from target String by regex
  assertEquals("Helloworld,IamDavy", removedStr);

  removedStr = str.replaceAll("\\s+", ""); // Find space from target String by regex
  assertEquals("Helloworld,IamDavy", removedStr);
}

       StringUtils.remove source code:

public static String remove(final String str, final char remove) {
    if (isEmpty(str) || str.indexOf(remove) == INDEX_NOT_FOUND) {
        return str;
    }
    final char[] chars = str.toCharArray();
    int pos = 0;
    for (int i = 0; i < chars.length; i++) {
        if (chars[i] != remove) {
            chars[pos++] = chars[i];
        }
    }
    return new String(chars, 0, pos);
}

    5> How to reverse String?

@Test
public void reverseStringTest() {
  String str = "Hello world, I am  Davy";
  String reversedStr = StringUtils.reverse(str);
  assertEquals("yvaD  ma I ,dlrow olleH", reversedStr);

  reversedStr = new StringBuilder(str).reverse().toString();
  assertEquals("yvaD  ma I ,dlrow olleH", reversedStr);

  // Or we can use Stack<Character> to reverse a String
  Stack<Character> stack = new Stack<Character>();
  for (char c : str.toCharArray()) {
      stack.push(c);
  }
  StringBuilder sb = new StringBuilder();
  while (!stack.empty()) {
      sb.append(stack.pop());
  }
  reversedStr = sb.toString();
  assertEquals("yvaD  ma I ,dlrow olleH", reversedStr);
}
       StringUtils.reverse source code:
public static String reverse(final String str) {
    if (str == null) {
        return null;
    }
    return new StringBuilder(str).reverse().toString();
}

 

Char Operation:

    1> How to recognize a char is upper case or lower case?

@Test
public void caseTest() {
  char c = 'A';
  assertTrue(Character.isUpperCase(c));

  c = 'a';
  assertTrue(Character.isLowerCase(c));

  c = '0';
  assertFalse(Character.isUpperCase(c));
  assertFalse(Character.isLowerCase(c));
}

    2> How to swap case for char?

@Test
public void swapCaseTest() {
  char c = 'A';
  assertEquals('a', Character.toLowerCase(c));

  c = 'a';
  assertEquals('A', Character.toUpperCase(c));

  c = '0';
  assertEquals('0', Character.toUpperCase(c));
  assertEquals('0', Character.toLowerCase(c));
}

 

File Operation:

    1> How to create temp file simply? -Use File.createTempFile();

public static File createTempFile(String prefix, String suffix, File directory);

import java.io.File;
import java.io.IOException;

import org.apache.commons.io.FileUtils;
import org.junit.Test;

public class FileTest {
    @Test
    public void createTempFileTest() throws IOException {
	File tmpFile = File.createTempFile("test", ".tmp");
	File srcFile = new File("src/main/resources/spring.xml");
	FileUtils.copyFile(srcFile, tmpFile, false);
    }
}
// The tmpFile would be C:/Users/userID/AppData/Local/Temp/test3900920229769745162.tmp

    2> How to copy file simply? -Use FileUtils.copyFile();

    3> How to close I/O stream? -Use IOUtils.closeQuitely();

 

Java Date Format: 

    1> java.text.SimpleDateFormat is not thread safe, use org.apache.commons.lang.time.FastDateFormat instead.

     2> "yyyyMMdd" is Year-Month-Day, "MM" is "Month" and "mm" is "Minutes".

 

Java Reflect:

    1> How to know if a class is primitive type or not?

Class<?> clazz = Integer.class;
Class<?> clazz2 = int.class;
if (clazz.isPrimitive()) {
    System.out.println("PRIMITIVE");
}else{
    System.out.println("NON_PRIMITIVE");
}

 

 

Reference Links:

1) http://stackoverflow.com/questions/9698652/subtraction-of-two-sets-in-java

2) http://stackoverflow.com/questions/5455794/removing-whitespace-from-strings-in-java

 

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