How to write Unit test, branch test, condition, branch and relational operator, definition use criteria for class writen in JAVA

I have to write unit test, branch test, condition.... but i have no idea how to do that Can someone help me ? I am beginner and i really don't understand testing... I read some simple examples given by the professor but this one...

class IntegerArray {

    private int a[];

    public IntegerArray(int a[]) {
        this.a = Arrays.copyOf(a, a.length);
    }

    private IntegerArray(int a[],boolean to_copy) {
        if ( to_copy )
            this.a = a;
        else
            this.a = Arrays.copyOf(a, a.length);
    }

    public int length() {
        return a.length;
    }

    public int getElementAt(int i) {
         return a[i];
    }

    public int sum() {
        int sum = 0;
        for ( int e : a ) sum += e;
        return sum;
    }

    public double average() {
        return sum()*1.0/length();
    }

    public IntegerArray getSorted() {
        int sorted_a[] = Arrays.copyOf(a, length());
        Arrays.sort(sorted_a);
        return new IntegerArray(sorted_a);
    }

    public IntegerArray concat(IntegerArray ia) {
        int res_a[] = new int[a.length+ia.a.length];
        System.arraycopy(a, 0, res_a, 0, a.length);
        System.arraycopy(ia.a, 0, res_a, a.length, ia.a.length);
        return new IntegerArray(res_a,true);
    }

public String toString() {
        return Arrays.toString(a);
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + Arrays.hashCode(a);
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        IntegerArray other = (IntegerArray) obj;
        if (!Arrays.equals(a, other.a))
            return false;
        return true;
    }   
}