Improve the Quality of Java Code

This tutorial describes some rules and tools aimed at improving the quality of a Java code to simplify its maintenance. It makes up a minimum consistent set of rules which can be applied in any situation, especially on embedded systems where performance and low memory footprint matter.

Intended Audience

The audience for this document is engineers who are developing any kind of Java code (application or library).

Readable Code

This section describes rules to get a readable code, in order to facilitate:

  • the maintenance of an existing code with multiple developers contributions (e.g. merge conflicts, reviews).
  • the landing to a new code base when the same rules are applied across different modules and components.

Naming Convention

Naming of Java elements (package, class, method, field and local) follows the Camel Case convention.

  • Package names are written fully in lower case (no underscore).
  • Package names are singular (e.g. ej.animal instead of ej.animals).
  • Class names are written in upper camel case.
  • Method and instance field names are written in lower camel case.
  • Static field names are written in lower camel case.
  • Constant names are written in fully upper case with underscore as word separator.
  • Enum constant names are written in fully upper case with underscores as word separators.
  • Local (and parameter) names are written in lower camel case.
  • When a name contains an acronym, capitalize only the first letter of the acronym (e.g. for a local with the HTTP acronym, use myHttpContext instead of myHTTPContext).

It is also recommended to use full words instead of abbreviations (e.g. MyProxyReference instead of MyProxyRef).

Visibility

Here is a list of the usage of each Java element visibility:

  • public: API.
  • protected: API for subclasses.
  • package: component intern API (collaboration inside a package).
  • private: internal structure, cache, lazy, etc.

By default, all instance fields must be private.

Package visibility can be used by writing the comment /*default*/ in place of the modifier.

Javadoc

Javadoc comments convention is based on the official documentation.

Note

Javadoc is written in HTML format and doesn’t accept XHTML format: tags must not be closed. For example, use only a <p> between two paragraphs.

Here is a list of the rules to follow when writing Javadoc:

  • All APIs (see Visibility) must have a full Javadoc (classes, methods, and fields).
  • Add a dot at the end of all phrases.
  • Add @since tag when introducing a new API.
  • Do not hesitate to use links to help the user to navigate in the API (@see, @link).
  • Use the @code tag in the following cases:
    • For keywords (e.g. {@code null} or {@code true}).
    • For names and types (e.g. {@code x} or {@code Integer}).
    • For code example (e.g. {@code new Integer(Integer.parseInt(s))}).

Here is a list of additional rules for methods:

  • The first sentence starts with the third person (as if there is This method before).
  • All parameters and returned values must be described.
  • Put as much as possible information in the description, keep @param and @return minimal.
  • Start @param with a lower case and usually with the or a.
  • Start @return with a lower case as if the sentence starts with Returns.
  • Avoid naming parameters anywhere other than in @param. If the parameter is renamed afterward, the comment is not changed automatically. Prefer using the given xxx.

Code Style and Formatting

MicroEJ defines a formatting profile for .java files, which is automatically set up when creating a new Module Project Skeleton.

Note

MicroEJ SDK automatically applies formatting when a .java file is saved. It is also possible to manually apply formatting on specific files:

  • In Package Explorer, select the desired files, folders or projects,
  • then go to Source > Format. The processed files must not have any warning or error.

Here is the list of formatting rules included in this profile:

  • Indentation is done with 1 tab.
  • Braces are mandatory with if, else, for, do, and while statements, even when the body is empty or contains only a single statement.
  • Braces follow the Kernighan and Ritchie style (Egyptian brackets) described below:
    • No line break before the opening brace.
    • Line break after the opening brace.
    • Line break before the closing brace.
    • Line break after the closing brace, only if that brace terminates a statement or terminates the body of a method, constructor, or named class. For example, there is no line break after the brace if it is followed by else or a comma.
  • One statement per line.
  • Let the formatter automatically wraps your code when a statement needs to be wrapped.

Here is a list of additional formatting rules that are not automatically applied:

  • Class and member modifiers, when present, must appear in the order recommended by the Java Language Specification: public protected private abstract default static final transient volatile synchronized native strictfp.
  • Avoid committing commented code (other than to explain an optimization).
  • All methods of an interface are public. There is no need to specify the visibility (easier to read).
  • The parts of a class or interface declaration must appear in the order suggested by the Code Convention for the Java Programming Language:
    • Class (static) fields. First, the public class fields, then the protected, then package level (no access modifier), and then the private.
    • Instance fields. First, the public class fields, then the protected, then package level (no access modifier), and then the private.
    • Constructors
    • Methods

Note

Most of these rules are checked by Code Analysis with SonarQube™.

Best Practices

This section describes rules made of best practices and well-known restrictions of the Java Programming Language and more generally Object Oriented paradigm.

Common Pitfalls

  • Object.equals(Object) and Object.hashCode() methods must be overridden in pairs. See Equals and Hashcode.
  • Do not assign fields in field declaration but in the constructor.
  • Do not use non-final method inside the constructor.
  • Do not overburden the constructor with logic.
  • Do not directly store an array given by parameter.
  • Do not directly return an internal array.
  • Save object reference from a field to a local before using it (see Local Extraction).

Simplify Maintenance

  • Extract constants instead of using magic numbers.
  • Use parenthesis for complex operation series; it simplifies the understanding of operator priorities.
  • Write short lines. This can be achieved by extracting locals (see Local Extraction).
  • Use a limited number of parameters in methods (or perhaps a new type is needed).
  • Create small methods with little complexity. When a method gets too complex, it should be split.
  • Use + operator only for single-line string concatenation. Use an explicit StringBuilder otherwise.
  • Use component-oriented architecture to separate concerns. If a class is intended to be instantiated using Class.newInstance(), add a default constructor (without parameters).

Basic Optimizations

  • Avoid explicitly initializing fields to 0 or null, because they are zero-initialized by the runtime. A //VM_DONE comment can be written to understand the optimization.
  • The switch/case statements are generated by the Java compiler in two ways depending on the cases density. Prefer declaring consecutive cases (table_switch) for performance (O(1)) and slightly smaller code memory footprint instead of lookup_switch (O(log N)).
  • Avoid using built-in thread safe types (Vector, Hashtable, StringBuffer, etc.). Usually synchronization has to be done at a higher level.
  • Avoid serializing/deserializing data from byte arrays using manual bitwise operations, use ByteArray utility methods instead.

Local Extraction

Local extraction consists of storing the result of an expression before using it, for example:

Object myLocale = this.myField;
if (myLocale != null) {
  myLocale.myMethod();
}

It improves the Java code in many ways:

  • self documentation: gives a name to a computed result.
  • performance and memory footprint: avoids repeated access to same elements and extract loop invariants.
  • thread safety: helps to avoid synchronization issues or falling into unwanted race conditions.
  • code pattern detection: helps automated tools such as Null Analysis.

Equals and Hashcode

The purpose of these methods is to uniquely and consistently identify objects. The most common use of these methods is to compare instances in collections (list or set elements, map keys, etc.).

The Object.equals(Object) method implements an equivalence relation (defined in the Javadoc) with the following properties:

  • It is reflexive: for any reference value x, x.equals(x) must return true.
  • It is symmetric: for any reference values x and y, x.equals(y) must return true if and only if y.equals(x) returns true.
  • It is transitive: for any reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) must return true.
  • It is consistent: for any reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the object is modified.
  • For any non-null reference value x, x.equals(null) must return false.

Avoid overriding the equals(Object) method in a subclass of a class that already overrides it; it could break the contract above. See Effective Java book by Joshua Bloch for more information.

If the equals(Object) method is implemented, the hashCode() method must also be implemented. The hashCode() method follows these rules (defined in the Javadoc):

  • It must consistently return the same integer when invoked several times.
  • If two objects are equal according to the equals(Object) method, then calling the hashCode() method on each of the two objects must produce the same integer result.
  • In the same way, it should return distinct integers for distinct objects.

The equals(Object) method is written that way:

  • Compare the argument with this using the == operator. If both are equals, return true. This test is for performance purposes, so it is optional and may be removed if the object has a few fields.
  • Use an instanceof to check if the argument has the correct type. If not, return false. This check also validates that the argument is not null.
  • Cast the argument to the correct type.
  • For each field, check if that field is equal to the same field in the casted argument. Return true if all fields are equal, false otherwise.
@Override
public boolean equals(Object o) {
  if (o == this) {
    return true;
  }
  if (!(o instanceof MyClass)) {
    return false;
  }
  MyClass other = (MyClass)o;
  return field1 == other.field1 &&
    (field2 == null ? other.field2 == null : field2.equals(other.field2));
}

The Object.hashCode() method is written that way:

  • Choose a prime number.
  • Create a result local, whatever the value (usually the prime number).
  • For each field, multiply the previous result with the prime plus the hash code of the field and store it as the result.
  • Return the result.

Depending on its type, the hash code of a field is:

  • Boolean: (f ? 0 : 1).
  • Byte, char, short, int: (int) f).
  • Long: (int)(f ^ (f >>> 32)).
  • Float: Float.floatToIntBits(f).
  • Double: Double.doubleToLongBits(f) and the same as for a long.
  • Object: (f == null ? 0 : f.hashCode()).
  • Array: add the hash codes of all its elements (depending on their type).
private static final int PRIME = 31;

@Override
public int hashCode() {
  int result = PRIME;
  result = PRIME * result + field1;
  result = PRIME * result + (field2 == null ? 0 : field2.hashCode());
  return result;
}