Top 10 Mistakes Java Developers Make

check-if-an-array-contains-a-value-right.java

Arrays.asList(arr).contains(targetValue);

// OR

for(String s: arr){
	if(s.equals(targetValue))
		return true;
}
return false;

check-if-an-array-contains-a-value-wrong.java

Set<String> set = new HashSet<String>(Arrays.asList(arr));
return set.contains(targetValue);

constructor-of-super-and-sub-right.java

public Super() {
  System.out.println("Super");
}

constructor-of-super-and-sub-wrong.java

class Super {
  String s;
  
  public Super(String s) {
    this.s = s;
  }
}

public class Sub extends Super {
  int x = 200;
  public Sub(String s) { // error here
    
  }
  
  public Sub() { // error here
    System.out.println("Sub");
  }
  
  public static void main(String[] args) {
    Sub s = new Sub();
  }
}

convert-array-to-arraylist-right.java

ArrayList<String> arrayList = new ArrayList<String>(Arrays.asList(arr));

convert-array-to-arraylist-wrong.java

List<String> list = Arrays.asList(arr);

mutable-vs-immutable.java

String result="";
for(String s: arr){
	result = result + s;
}

quote-or-constructor-1.java

//1. use double quotes
String x = "abc";
//2. use constructor
String y = new String("abc");

quote-or-constructor-2.java

String a = "abcd";
String b = "abcd";
System.out.println(a == b);  // True
System.out.println(a.equals(b)); // True
 
String c = new String("abcd");
String d = new String("abcd");
System.out.println(c == d);  // False
System.out.println(c.equals(d)); // True

remove-an-element-from-a-list-inside-a-loop-right.java

ArrayList<String> list = new ArrayList<String>(Arrays.asList("a", "b", "c", "d"));
Iterator<String> iter = list.iterator();
while (iter.hasNext()) {
	String s = iter.next();
 
	if (s.equals("a")) {
		iter.remove();
	}
}

remove-an-element-from-a-list-inside-a-loop-wrong1.java

ArrayList<String> list = new ArrayList<String>(Arrays.asList("a", "b", "c", "d"));
for (int i = 0; i < list.size(); i++) {
	list.remove(i);
}
System.out.println(list);

// output
// [b, d]

remove-an-element-from-a-list-inside-a-loop-wrong2.java

ArrayList<String> list = new ArrayList<String>(Arrays.asList("a", "b", "c", "d"));
 
for (String s : list) {
	if (s.equals("a"))
		list.remove(s);
}

use-raw-type-of-collection.java

public static void add(List list, Object o){
	list.add(o);
}
public static void main(String[] args){
	List<String> list = new ArrayList<String>();
	add(list, 10);
	String s = list.get(0);
}

// Exception will be thrown..
// Exception in thread "main" java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
// 	at ...