EECS2030 Sample Test 2

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.TreeMap;

/**
 * A utility class for Test 2.
 * 
 * @author EECS2030
 * 
 */
public class Test2D {

	private Test2D() {}
	
	/**
	 * Returns the string at the end of the list t
	 * 
	 * @param t
	 *            a list
	 * @pre. t.size() > 0
	 * @return the string at the end of t
	 */
	public static String last(List<String> t) {
		return t.get(t.size() - 1);
	}

	/**
	 * Returns the second last string in the list t
	 * 
	 * @param t
	 *            a list
	 * @return the second last string in t
	 * @throws IllegalArgumentException
	 *             if the length of the list is less than 2
	 */
	public static String secondLast(List<String> t) {
		if (t.size() < 2) {
			throw new IllegalArgumentException("list must have length of 2");
		}
		return t.get(t.size() - 2);
	}

	/**
	 * Returns the string formed by concatenating all of the strings contained
	 * in the list t. The strings are concatenated sequentially starting from
	 * the first string in the list and ending with the last string in the list.
	 * For example, if t is the list:
	 * 
	 * <p>
	 * ["he", "ll", "o"]
	 * 
	 * <p>
	 * then the resulting string is "hello"
	 * 
	 * @param t
	 *            a list of strings to concatenate
	 * @return the concatenation of the strings in t
	 */
	public static String concat(List<String> t) {
		StringBuilder b = new StringBuilder();
		for (String s : t) {
			b.append(s);
		}
		return b.toString();
	}


	/**
	 * Sorts a list of strings by their length (shortest first). Strings with
	 * the same length are sorted in dictionary order. For example, if the list
	 * t is:
	 * 
	 * ["abstract", "boolean", "char", "code", "for"]
	 * 
	 * then t is modified so that it becomes:
	 * 
	 * ["for", "char", "code", "boolean", "abstract"]
	 * 
	 * @param t
	 *            the list to sort
	 */
	public static void sortByLength(List<String> t) {
		Map<Integer, List<String>> lengths = new TreeMap<Integer, List<String>>();
		for (String s : t) {
			int sLen = s.length();
			List<String> u = lengths.get(sLen);
			if (u == null) {
				u = new ArrayList<String>();
				lengths.put(sLen, u);
			}
			u.add(s);
		}
		t.clear();
		for (List<String> u : lengths.values()) {
			Collections.sort(u);
			t.addAll(u);
		}
	}

}

Multiple choice question

public class Student {
  private String studentNumber;
  
  public Student(long number) {
    this.studentNumber = new String(Long.toString(number));
  }

  @Override
  public boolean equals(Object obj) {
    if (this == obj) {
      return true;
    }
    if (this.getClass() != obj.getClass()) {
      return false;
    }
    Student other = (Student) obj;
    return this.studentNumber == other.studentNumber;
  }
  
  // hashCode not shown, but guaranteed to be correct
}

Student s = new Student(123);
Student t = new Student(123);
System.out.println(s.equals(t));