Avoid creating unnecessary objects and always prefer to do Lazy Initialization
Object creation in Java is one of the most expensive operation in terms of memory utilization and performance impact. It is thus advisable to create or initialize an object only when it is required in the code.
public class Countries {
private List countries;
public List getCountries() {
//initialize only when required
if(null == countries) {
countries = new ArrayList();
}
return countries;
}
}
Here's an example of code that would trigger this rule:
public class Foo {
private void foo(String bar) {
bar = "something else";
}
}
Avoid Reassigning Parameters
Reassigning values to parameters is a questionable practice. Use a temporary local variable instead or use only when required.Here's an example of code that would trigger this rule:
public class Foo {
private void foo(String bar) {
bar = "something else";
}
}
Loose Coupling
Avoid using implementation types (i.e., HashSet); use the interface (i.e, Set) instead.Here's an example of code that would trigger this rule:
import java.util.*;
public class Bar {
// Use List instead
private ArrayList list = new ArrayList();
// Use Set instead
public HashSet getFoo() {
returnnew HashSet();
}
}
Null Assignment
Assigning a "null" to a variable (outside of its declaration) is usually bad form. Some times, the assignment is an indication that the programmer doesn't completely understand what is going on in the code.NOTE: This sort of assignment may in rare cases be useful to encourage garbage collection. If that's what you're using it for, by all means, disregard this rule :-)
Here's an example of code that would trigger this rule:
public class Foo {
public void bar() {
Object x = null; // This is OK.
x = new Object();
// Big, complex piece of code here.
x = null; // This is BAD.
// Big, complex piece of code here.
}
}
Never make an instance fields of class public
Making a class field public can cause lot of issues in a program. For instance you may have a class called MyCalender. This class contains an array of String weekdays. You may have assume that this array will always contain 7 names of weekdays. But as this array is public, it may be accessed by anyone. Someone by mistake also may change the value and insert a bug!
public class MyCalender {
public String[] weekdays =
{"Sun", "Mon", "Tue", "Thu", "Fri", "Sat", "Sun"};
//some code
}
Best approach as many of you already know is to always make the field private and add a getter method to access the elements.
private String[] weekdays =
{"Sun", "Mon", "Tue", "Thu", "Fri", "Sat", "Sun"};
public String[] getWeekdays() {
return weekdays;
}
But writing getter method does not exactly solve our problem. The array is still accessible. Best way to make it unmodifiable is to return a clone of array instead of array itself. Thus the getter method will be changed to.
public String[] getWeekdays() {
return weekdays.clone();
}
Here's an example of code that would violate this rule:
public final class Bar {
private int x;
protected int y; // Bar cannot be subclassed, so is y really
// private or package visible???
Bar() {}
}
Here's an example of code that would violatethis rule:
class Foo {
boolean bar(String x) {
return x.equals("2"); // should be "2".equals(x)
}
}
Here's an example of code that would violate this rule:
public class Foo {
// Try to avoid this
synchronized void foo() {
}
// Prefer this:
void bar() {
synchronized(this) {
}
}
}
Avoid Protected Field In Final Class
Do not use protected fields in final classes since they cannot be subclassed. Clarify your intent by using private or package access modifiers instead.Here's an example of code that would violate this rule:
public final class Bar {
private int x;
protected int y; // Bar cannot be subclassed, so is y really
// private or package visible???
Bar() {}
}
Position Litera ls First In Comparisons
Position literals first in String comparisons - that way if the String is null you won't get a NullPointerException, it'll just return false.Here's an example of code that would violatethis rule:
class Foo {
boolean bar(String x) {
return x.equals("2"); // should be "2".equals(x)
}
}
Avoid Synchronized At Method Level
Method level synchronization can backfire when new code is added to the method. Block-level synchronization helps to ensure that only the code that needs synchronization gets it.Here's an example of code that would violate this rule:
public class Foo {
// Try to avoid this
synchronized void foo() {
}
// Prefer this:
void bar() {
synchronized(this) {
}
}
}
Always try to minimize Mutability of a class
Making a class immutable is to make it unmodifiable. The information the class preserve will stay as it is through out the lifetime of the class. Immutable classes are simple, they are easy to manage. They are thread safe. They makes great building blocks for other objects.
However creating immutable objects can hit performance of an app. So always choose wisely if you want your class to be immutable or not. Always try to make a small class with less fields immutable.
To make a class immutable you can define its all constructors private and then create a public static method
to initialize and object and return it.
public class Employee {
private String firstName;
private String lastName;
//private default constructor
private Employee(String firstName, String lastName) {
this.firstName = firstName;
this.lastName = lastName;
}
public static Employee valueOf (String firstName, String lastName) {
return new Employee(firstName, lastName);
}
}
public class Employee {
private String firstName;
private String lastName;
//private default constructor
private Employee(String firstName, String lastName) {
this.firstName = firstName;
this.lastName = lastName;
}
public static Employee valueOf (String firstName, String lastName) {
return new Employee(firstName, lastName);
}
}
Try to prefer Interfaces instead of Abstract classes
First you can not inherit multiple classes in Java but you can definitely implements multiple interfaces. Its very easy to change the implementation of an existing class and add implementation of one more interface rather then changing full hierarchy of class.
Again if you are 100% sure what methods an interface will have, then only start coding that interface. As it is very difficult to add a new method in an existing interface without breaking the code that has already implemented it. On contrary a new method can be easily added in Abstract class without breaking existing functionality.
Always try to limit the scope of Local variable
Local variables are great. But sometimes we may insert some bugs due to copy paste of old code. Minimizing the scope of a local variable makes code more readable, less error prone and also improves the maintainability of the code.
Thus, declare a variable only when needed just before its use.
Always initialize a local variable upon its declaration. If not possible at least make the local instance assigned null value.
Try to use standard library instead of writing your own from scratch
Writing code is fun. But “do not reinvent the wheel”. It is very advisable to use an existing standard library which is already tested, debugged and used by others. This not only improves the efficiency of programmer but also reduces chances of adding new bugs in your code. Also using a standard library makes code readable and maintainable.
For instance Google has just released a new library Google Collections that can be used if you want to add advance collection functionality in your code.
Wherever possible try to use Primitive types instead of Wrapper classes
Wrapper classes are great. But at same time they are slow. Primitive types are just values, whereas Wrapper classes are stores information about complete class.
Sometimes a programmer may add bug in the code by using wrapper due to oversight. For example, in below example:
int x = 10;
int y = 10;
Integer x1 = new Integer(10);
Integer y1 = new Integer(10);
System.out.println(x == y);
System.out.println(x1 == y1);
The first sop will print true whereas the second one will print false. The problem is when comparing two wrapper class objects we cant use == operator. It will compare the reference of object and not its actual value.
Also if you are using a wrapper class object then never forget to initialize it to a default value. As by default all wrapper class objects are initialized to null.
Boolean flag;
if(flag == true) {
System.out.println("Flag is set");
} else {
System.out.println("Flag is not set");
}
The above code will give a NullPointerException as it tries to box the values before comparing with true and as its null.
Use Strings with utmost care.
Always carefully use Strings in your code. A simple concatenation of strings can reduce performance of program. For example if we concatenate strings using + operator in a for loop then everytime + is used, it creates a new String object. This will affect both memory usage and performance time.
Also whenever you want to instantiate a String object, never use its constructor but always instantiate it directly. For example:
//slow instantiation
String slow = new String("Yet another string object");
//fast instantiation
String fast = "Yet another string object";
Use String Buffer For String Appends
Finds usages of += for appending strings.
Here's an example of code that would violate this rule:
Public class Foo {
void bar() {
String a;
a = "foo";
a += " bar";
// better would be:
// StringBuffer a = new StringBuffer("foo");
// a.append(" bar);
}
}
String Instantiation
Avoid instantiating String objects; this is usually unnecessary.
Here's an example of code that would violate this rule:
public class Foo {
private String bar = new String("bar"); // just do a String bar = "bar";
}
Always return empty Collections and Arrays instead of null
Whenever your method is returning a collection element or an array, always make sure you return empty array/collection and not null. This will save a lot of if else testing for null elements. For instance in below example we have a getter method that returns employee name. If the name is null it simply return blank string “”.
public String getEmployeeName() {
return (null==employeeName ? "": employeeName);
}
Defensive copies are savior
Defensive copies are the clone objects created to avoid mutation of an object. For example in below code we have defined a Student class which has a private field birth date that is initialized when the object is constructed.
public class Student {
private Date birthDate;
public Student(birthDate) {
this.birthDate = birthDate;
}
public Date getBirthDate() {
return this.birthDate;
}
}
Now we may have some other code that uses the Student object.
public static void main(String []arg) {
Date birthDate = new Date();
Student student = new Student(birthDate);
birthDate.setYear(2019);
System.out.println(student.getBirthDate());
}
In above code we just created a Student object with some default birthdate. But then we changed the value of year of the birthdate. Thus when we print the birth date, its year was changed to 2019!
To avoid such cases, we can use Defensive copies mechanism. Change the constructor of Student class to following.
public Student(birthDate) {
this.birthDate = new Date(birthDate);
}
This ensure we have another copy of birthdate that we use in Student class.
Never let exception come out of finally block
Finally blocks should never have code that throws exception. Always make sure finally clause does not throw exception. If you have some code in finally block that does throw exception, then log the exception properly and never let it come out :)
Never throw “Exception”
Never throw java.lang.Exception directly. It defeats the purpose of using checked Exceptions. Also there is no useful information getting conveyed in caller method.
Sources: https://sites.google.com/site/javatouch/javacode-bestpractices
