# Backend Interview Exam

## 1、How does stack and heap work in java ?
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

## 2、What is Garbage Collection , How it work , Explain one implementation at leaste.

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

## 3、Please Explain Pessimistic Lock and Optimistic Lock.

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

## 4、By previous question, this code is present Pessimistic Lock. make it complete. tip's relpace #1 #2

/*
 * table: bank_account 
 * pk: bank_id (int64)
 * bank_id : bank serial number
 * 
 * table: product_stock
 * pk: product_id
 * product_id: bank serial number
 * number: it's a product price
 * 
 * */
public boolean consume(Long bankId, Integer cost){ 
    //locked bank account 
    BankAccount product = query("SELECT * FROM bank_account WHERE bank_id=#{bankId} #1", 
                                bankId);
    int productId = product.productId;
    if(product.getNumber() > 0) { 
    int updateCnt = 
        update("#2 product_stock " +
               "SET number=#{cost} WHERE product_id=#{productId}", cost, productId);
        if(updateCnt > 0){ 
            //update stock num successful
            return true;
        } 
    }
    return false; 
}

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

## 5、Given an integer x, return true if x is palindrome integer. An integer is a palindrome when it reads the same backward as forward.
For example, 121 is a palindrome while 123 is not.