Common problem

Java pass-by-value

1. reassignment

int[] a = new int[]{1}; // a is on stack
int[] b = a; // b still store on stack
b = new int[]{2};
// if print a, it will output 1

int[] a = new int[]{1}; // a is on stack
int[] b = a; // b still store on stack
b[0] = 2;
// if print a, it will output 2

1. pass in as parameter

// anything "new" in function will stay local and will not modify the outter scope variable
public void reassign(int[] array) {
    array = new int[]{2};
}

public void modify(int[] array) {
    array[0] = 2;
}

public int[] returnArray(int[] array) {
    array = new int[]{2};
    return array;
}

public void foo(int[] array) {
    array[0] = 2; // output this value 2
    array = new int[]{3};
    array[0] = 4;
}

public void foo2(int[] array) {
    array[0] = 2;
    array[0] = 4; // output 4
    array = new int[]{3};
}

public static void main(String[] args) {
    int[] array = new int[]{1};
    reassign(array); //what’s the result? 1
    modify(array); //what’s the result? 2 (ignore previous statement)
    array = returnArray(array); //what’s the result? 2 (ignore previous statement)
}

debug

public class Solution {
 public void reverse(int[] array) {
   int[] array1 = new int[array.length];
   for (int i=0; i<array.length; i=i+1) {
    array1[i]=array[array.length-1-i];
   }
   array = array1; // Write your solution here
 }

Problem: Give an array list of integer, calculate the sum of squares of all its elements. Note: return 0 if the list is null or empty.

Example: list = {1,2,3} → returns 14 (14=11+22+3*3)

public class Solution {
 public int sumOfSquare(List<Integer> list) {
    if (list == null || list.isEmpty()) {
      return 0;
    }
    int sum = 0;
    for (int i = 0; i < list.size(); i++) {
        sum += list.get(i) * list.get(i);
    }
    return sum;
}

Misc

1. String s = 4 + "aa"; // will print 4aa, because 4.toString() + "aa", however it won't work in c++
2. low -> high precision: will cast implicitly (no need to add (long) for example).
3. high -> low precision: need explict cast

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Last update: August 17, 2020