Java nullpointerexception issue

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Java

Ok, So I thought I understood nullpointerexception when I was working on C++, but now that I am making this class in Java, I obviously didn't understand as much as I thought because I am so confused what I did so wrong. I have changed 20 things and back and can't figure it out. This is the original version I had, can anyone tell me where I am making the error in my code?

<pre>
public class HamadHamdi {

	static MyString s1, s2, s3, s4;
	static void setup() {
		char[] c1 = {'h', 'e', 'l', 'l', 'o'};
		char[] c2 = {'h', 'e', 'l'};
		char[] c3 = {'H'};
		char[] c4 = {};
		s1 = new MyString(c1);
		s2 = new MyString(c2);
		s3 = new MyString(c3);
		s4 = new MyString(c4);
	}
	static void assertTrue(boolean b) {
		if (!b) {
			throw new AssertionError();
		}
	}
	static void assertFalse(boolean b) {
		if (b) {
			throw new AssertionError();
		}
	}
	static void testLength() {
		assertTrue(s1.length() == 5);
		assertTrue(s2.length() == 3);
		assertTrue(s3.length() == 1);
		assertTrue(s4.length() == 1);
	}
	static void testEquals() {
		Integer a = new Integer(123);
		char c[] = {'h', 'e', 'l'};
		MyString s = new MyString(c);
		assertTrue(s.equals(s2));
		assertFalse(s1.equals(s2));
		assertFalse(s3.equals(a));
	}
	public static void main(String[] args) {
		testLength();
		testEquals();
	}
}
class MyString implements Comparable<MyString> {
	private char[] data;
	
	public MyString(char[] chars) {
		this.data = new char[chars.length];
		for (int i = 0; i < chars.length; i++) {
			this.data[i] = chars[i];
		}
	}
	public char charAt(int i) {
		return data[i];
	}
	public int compareTo(MyString other) {
		int limit = Math.min(other.length(), length());

        char[] tempArr = other.data;
        int i = 0;
        while (i < limit) {
            char ch1 = charAt(i);
            char ch2 = tempArr[i];
            if (ch1 != ch2) {
                return ch1 - ch2;
            }
            i++;
        }

        return length() - other.length();
	}
	public boolean equals(MyString other) {
		boolean isEqual = true;
		if (other.length() != data.length)
			isEqual = false;
		else {
			for (int i = 0; i < data.length; i++) {
				if (other.data[i] != this.data[i])
					isEqual = false;
			}
		}
		return isEqual;
	}
	public boolean equalsIgnoreCase(MyString other) {
		MyString otherLowerCase = other.toLowerCase();
		MyString thisLowerCase = new MyString(data).toLowerCase();
		return thisLowerCase.equals(otherLowerCase);
		
	}
	public int length() {
		return data.length;
	}
	public void output() {
		System.out.println(data);
	}
	public MyString substring(int begin) {
		return substring(begin, data.length);
	}
	public MyString substring(int begin, int end) {
		int length = end - begin;
		char[] tempArr = new char[length];
		for (int i = 0; i < (length); i++) {
			tempArr[i] = data[begin];
			begin++;
		}
		return new MyString(tempArr);
	}
	public MyString toLowerCase() {
		char[] tempArr = new char[data.length];
		for (int i = 0; i < data.length; i++) {
			if (data[i] >= 'A' && data[i] <= 'Z') {
				tempArr[i] = (char)(data[i] + 32);
			}
			else
				tempArr[i] = data[i];
		}
		return new MyString(tempArr);
	}
	public MyString toUpperCase() {
		char[] tempArr = new char[data.length];
		for (int i = 0; i < data.length; i++) {
			if (data[i] >= 'a' && data[i] <= 'z') {
				tempArr[i] = (char)(data[i] - 32);
			}
			else
				tempArr[i] = data[i];
		}
		return new MyString(tempArr);
	}
	public static MyString valueOf(int i) {
		char[] tempArr = Integer.toString(i).toCharArray();
		return new MyString(tempArr);
	}
}

What I have tried:

I tried changing the constructor, but I do not know if that is what I need to do. Any help is welcome.