Skip to main content

Java Core - Top 10 Questions Every Developer Should Know

#RandomlyPickedByMe

  • What is the difference between Javascript and Java?
  • Difference between StringBuilder and StringBuffer?
  • Why do I get "SomeType@a3fde" when I print my code?
  • Why is String immutable?
  • Why "equals" method when we have "==" operator?
  • Is List<Dog> a subclass of List<Animal>?
  • Why shouldn't we use raw type?
  • Is Java “pass-by-reference” or “pass-by-value”?
  • What's the advantage of a Java enum versus a class with public static final fields?
  • Why "double x = 0.1 + 0.2" and result of print(x) is 0.30000000000000004?

1. What is the difference between Javascript and Java?

Holy crap! (Vietnamese: Thế quái nào lại có câu hỏi ngớ ngẩn vậy chứ?)

"Java and Javascript are similar like Car and Carpet are similar." - Greg Hewgill (on StackOverflow)

2. Difference between StringBuilder and StringBuffer

String is immutable. StringBuilder and StringBuffer are mutable. StringBuffer is thread-safe. StringBuilder is modern than StringBuffer.

"As of release JDK 5, StringBuffer class has been supplemented with an equivalent class designed for use by a single thread, StringBuilder. The StringBuilder class should generally be used in preference to this one, as it supports all of the same operations but it is faster, as it performs no synchronization."
http://docs.oracle.com/javase/8/docs/api/java/lang/StringBuffer.html

3. Why do I get "SomeType@a3fde" when I print my code?

Because all classes extend the class Object which has the method toString().
public String toString(){
 return getClass().getName() + "@" + Integer.toHexString(hasCode());
}

4. Why is String immutable?

String s = "Hello";
s.replace("e", "E");
System.out.println(s); //"Hello"

s is only a reference to an String object "Hello";

5. Why "equals" method when we have "==" operator?

"equals" can be overridden, but "==" not.

6. Is List<Dog> a subclass of List<Animal>?

No. Because a List<Dog> is not a List<Animal>.

7. Why shouldn't we use raw type?

Runtime trouble, ClassCastException.

8. Is Java “pass-by-reference” or “pass-by-value”?

Java has only pass by value. A very simple example to validate this.
public void test() {
    MyClass obj = null;
    init(obj);
    //After calling init method, obj still points to null
    //this is because obj is passed as value and not as reference.
}
private void init(MyClass objVar) {
    objVar = new MyClass();
}

Imagine that likes the Shortcut (Reference) and the real file (object) in Windows. Shortcut can change content of its current point to but also can the reference to new file.

9. What's the advantage of a Java enum versus a class with public static final fields?

Consider the purpose! You should use enum types any time you need to represent a fixed set of constants such as the planets in our solar system. However, if we concerns technically the features between these; there are some more benefit when using enum: type-safe, singleton, switch case, built-in methods, etc.

10. Why "double x = 0.1 + 0.2" and result of print(x) is 0.30000000000000004?

Because double/float simply can't represent a number like 0.1.  This is caused by the way computers store floating-point numbers.
Detail: http://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html

Reference:
[1]. https://stackoverflow.com/tags/java/info
Labels:

Comments

Post a Comment

Popular posts from this blog

Multiple Inheritance of State and Implementation

Today, I was just curious about why an enum can not extend anything else. I took a look on the Oracle document here , and I found the answer is below: "All enums implicitly extend java.lang.Enum. Because a class can only extend one parent (see Declaring Classes), the Java language does not support multiple inheritance of state (see Multiple Inheritance of State, Implementation, and Type), and therefore an enum cannot extend anything else." I have been learned of it before. But, wait a sec...! Why Java does not support multiple inheritance of state? Since I have worked with other programming languages like C++, I was able to make a class extend some other classes. The short answer is to avoid the issues of multiple inheritance of state .  I wonder if other programming languages have these below terms but Java does. Multiple inheritance of state It is the ability to inherit fields from multiple classes. There is a problem and Java avoids it. "For exa...
Post a Comment
Read more

4 Remarkable Notes for JSF Apps Using HTML5

In the previous post , I've already shared with you how my team consults clients by using a HTML prototype. This post is about the used technologies for reusing the provided HTML template and communicating with backend. What is the issue when using HTML elements with Primefaces components? Primefaces is a great extension for developing JSF web apps. However, it is really frustrating in case we have to make it work with an existing HTML template. Why? - Primefaces has its own theme for styling. - Primefaces changes the HTML structure. Therefore, that would be a huge effort to use the Primefaces' components to replicate the elements of the HTML template; especially it is impossible for images drawing by " canvas " tag. That requires us to find a better approach. Since Java EE 7 (introducing JSF 2.2 included), it supports to use HTML5 elements . The idea is that JSF components don't effect the style and HTML structure, so we can easily reuse the provided HTM...
2 comments
Read more

Using Drools to Dynamically Manipulate Metadata of JSF Components

The post is just an approach to change metadata (e.g maxlength, required, etc) of JSF components (e.g. inputText, selectOneMenue, etc) by Drools. Project structure Tools being used Java version 1.8.0_131 Apache Maven 3.5.0 Apache Tomcat 8.0.16 Don't forget to configure your confidential information on  these following files: pom.xml, settings.xml (Maven) and tomcat-users.xml (Tomcat). For example: Source code https://github.com/vnnvanhuong/java_lab/tree/master/jsfdrools
Post a Comment
Read more

Monday vhandit #2

  Introduction to OpenLDAP directory service "A directory is a specialized database specially designed for searching and browsing, in addition to supporting basic lookup and update functions" A directory service can be local, providing a restricted context; or global, providing service to a much broader context. Curlie is a good example of a directory service. LDAP (Lightweight Directory Access Protocol) is a protocol for accessing directory services , specifically X.500-based directory services. OpenLDAP is an open-source implementation of LDAP. Writing system software: code comments I have read Clean Code (by Uncle Bob) and I thought that I should void comments since the code it explains its implementation itself. That is right but not always true. In this post, the author categorized the comments into 9 types. Only "trivial comments" and "backup comments" are the ones that should be avoided. I myself agree with "writing good comments is harder tha...
Post a Comment
Read more

Regex - Check a text without special characters but German, French

Special characters such as square brackets ([ ]) can cause an exception " java.util.regex.PatternSyntaxException " or something like this if we don't handle them correctly. I had met this issue. In my case, my customers want our application should allow some characters in German and French even not allow some special characters. The solution is that we limit the allowed characters by showing the validation message on GUI. For an instance, the message looks like the following: "This field can't contain any special characters; only letters, numbers, underscores (_), spaces and single quotes (') are allowed." I used Regular Expression to check it. For entering Germany and French, I actually don't have this type of keyboard, so I referred these sites: * German characters: http://german.typeit.org/ * French characters: http://french.typeit.org/ Here is my code: package vn.nvanhuong.practice; import java.util.regex.Matcher; import java.util...
Post a Comment
Read more