1b)Explain why class Q4 below is not thread safe. Re-write it to make it thread safe.

class Q4{

private boolean f[] = new boolean[20];

public Q4(){ for(int j = 0; j < 20; j++) f[j] = true;}

public void negate(int a, int b){ for(int j = a; j < b; j++)f[j] = !f[j]; }

public void swap(int a, int b){ boolean temp = f[a]; f[a] = f[b]; f[b] = temp; } }