Java Tips, Tricks and Pitfalls

Java
Tips, Tricks and Pitfalls

KISS - Keep it Short and Simple!

KISS - Example 1 (Read properties from file)
Example of properties file:
hibernate.connection.username=myuser
hibernate.connection.password=secret
hibernate.c3p0.min_size=5
hibernate.c3p0.max_size=20
hibernate.c3p0.timeout=1800
hibernate.c3p0.max_statements=50

KISS - Example 1 (Implementation)
Overthinking
Elegance
Laziness
Experience
VS

KISS - Example 1 (Overthinking)
Let’s create a method with really self-describing name!
public Properties readAllLinesFromFileAndReturnPropertiesObject(File file) {
Properties properties = null;
// logic will be here
return properties;
}

KISS - Example 1 (Elegance)
Let’s create a method with really self-describing name!
public Properties readProperties(String filename) {
// logic will be here
return properties;
}

Let’s read the file and fill the properties!
BufferedReader bufferedReaderForFileWhichShouldContainsLines = new BufferedReader(
new InputStreamReader(new BufferedInputStream(new FileInputStream(file)))
);
String accumulatorForLineFromFile = null; properties = new Properties();
while ((accumulatorForLineFromFile = brffwhcl.readLine()) != null)
{
properties.setProperty(accumulatorForLineFromFile.split("=")[0],
accumulatorForLineFromFile.split("=")[1]);
}

KISS - Example 1 (Experience)
Let’s read the file and fill the properties!
public Properties readProperties(String filename) {
Properties properties = new Properties();
properties.load(new FileInputStream(filename));
return properties;
}

Let’s handle some exceptions!
if (file != null) {
...
} else {
throw new IllegalArgumentException("file which should contains all needed information "
+
"to init Properties object is null, that means that Java can't work with it, "
+
"please check your code and make sure that you are passing valid arguments",
new NullPointerException()
);
}
return properties;
}

And several more...
if (file != null) {
if (file.exists()) {
if (file.canRead()) {
try {
BufferedReader
bufferedReaderForFileWhichShouldContainsLines = new BufferedReader(
new InputStreamReader(
new BufferedInputStream(
new FileInputStream(file)
)
)
);
try {
String accumulatorForLineFromFile = null;
properties = new Properties();
while ((accumulatorForLineFromFile
=
bufferedReaderForFileWhichShouldContainsLines.readLine()) != null) {
properties.setProperty(
accumulatorForLineFromFile.split("=")[0],
accumulatorForLineFromFile.split("=")[1]
);
}
} catch (IOException ioException) {
throw new IllegalArgumentException("file which should contains all needed information "
+
"to init Properties object is not null, but it isn't suitable for reading, " +
"that means that Java can't work with it, " +
new IOException(ioException)
);
} finally {
try {
bufferedReaderForFileWhichShouldContainsLines.close();
} catch (IOException ioException) {
System.err.printf("Can't close bufferedReaderForFileWhichShouldContainsLines: %s",
ioException.getMessage()
);
}
}
} catch (FileNotFoundException fileNotFoundException) {
throw new IllegalArgumentException("file which should contains all needed information " +
"to init Properties object is not null, but it not found, " +
new FileNotFoundException(fileNotFoundException.getMessage())
);
}
} else {
"to init Properties object is not null, but it now unreadable, " +
new IllegalAccessException()
);
}
} else {
"to init Properties object is not null, but it isn't exists, " +
new IllegalAccessException()
);
}
} else {
"to init Properties object is null, that means that Java can't work with it, " +
new NullPointerException()
);
}
return properties;
}

KISS - Example 1 (Laziness)
Let’s handle some exceptions!
public Properties readProperties(String filename) throws IOException {
Properties properties = new Properties();
properties.load(new FileInputStream(filename));
return properties;
}

Naming best & bad practices
Programming routine:

Naming best practices
● Keep all names close to context and self-describing
○ But avoid “secret knowledge”
○ Share the context via javadoc or comments

● Variable type should be obvious
○ That doesn’t mean that name should contain Type

● Boolean variables/methods names should be suitable
for the sentence: “if <variable_name> then ...?”

● Methods names should answer the question “what to do?”

● Methods names should answer the question “what to do?”
● Type names might contain parent names

Naming bad practices
● Using words that means nothing: obj, data, value, tmp...
a. There is always place for exceptions

● Fake/Wrong variable type
a. This might cost a burned chair...

● Single-character names: w, t, f ... o, m, g
a. Especially without any context - 100% burned chair

● Fake/Incomplete names of the methods

● Fake/Incomplete names of the methods
● Public argument names based on secret knowledge

Dirty and Pure methods
Hello, my name is side-effect...
public int[] sortArray(int[] array) {
Arrays.sort(array);
return array;
}

And sometimes I’m not obvious at all…
public <T> T[] sortArray(T[] array) {
if (array == null || array.length < 2) return array;
Comparator<T> comparator = null;
T[] tmpArray = checkAndReturnArray(array);
T notNullElement = null;
for (int i = 0; i < tmpArray.length && notNullElement == null; notNullElement = tmpArray[i++]) ;
if (notNullElement == null) return array;
if (notNullElement instanceof Comparator) { comparator = (Comparator<T>) notNullElement; }
else if (notNullElement instanceof Comparable) {
comparator = (o1, o2) -> {
if (o1 == null) return 1; if (o2 == null) return -1; return ((Comparable<T>) o1).compareTo(o2);
};
}
if (comparator == null) return array;
Arrays.sort(tmpArray, comparator);
return tmpArray;
}

Here's Johnny!
/**
* Checks array and returns copy if Mars is in the zenith of Saturn
*/
public <T> T[] checkAndReturnArray(T[] array) {
return (array != null)
? ((System.currentTimeMillis() > (42 * 64 * 30 * 365))
? ((System.getenv("magic") == null)
? array : null) : array) : null;
}

Immutability and final keyword
If you can’t change the methods,
you can change the arguments...

What is immutability?
class Immutable {
private final String immutableField;
public Immutable(String immutableField) {
this.immutableField = immutableField;
}
public String getImmutableField() {
return immutableField;
}
}
Examples:
String, Integer, File, URL...

What is immutability?
class Immutable {
private final String immutableField;
public Immutable(String immutableField) {
this.immutableField = immutableField;
}
public String getImmutableField() {
return immutableField;
}
}
Examples:
String, Integer, File, URL...
What is mutability?
class Mutable {
private List<String> mutableField;
public List<String> getMutableField() {
return mutableField;
}
public void setMutableField(List<String> mutableField) {
this.mutableField = mutableField;
}
}
Examples:
ArrayList, Arrays, StringBuilder...

Why immutability is good?
● Preventing side-effects
● Thread-safe
● Easy to debug
● Easy to implement
● Easy to extend
● Easy to scale

Why immutability is good?
● Preventing side-effects
● Thread-safe
● Easy to debug
● Easy to implement
● Easy to extend
● Easy to scale
Why immutability is bad?
● Real world isn’t immutable
● Constraints you
● Not always convenient
● Increases memory usage
● Performance degradation
● There is always will be an
exception...

Keyword final can be used with:
● Variables
○ final Object someField;

● Variables
● Methods
○ final public void someMethod() { ... }
○ public void someMethod(final Object argument) { ... }

● Variables
● Methods
○ final public void someMethod() { ... }
○ public void someMethod(final Object argument) { ... }
● Classes
○ public final class Example3;

try-catch-finally pitfalls
Simple puzzle?
public static int puzzle() {
try {
return 5;
} catch (Exception ex) {
return 10;
} finally {
return 15;
}
}

try pitfalls
● It’s a new scope…
● Each line might raise exception
● catch or finally is required
● Try with resources?
○ Custom AutoCloseable implementation
○ Implicit side-effect

catch pitfalls
● Type erasing
● Performance degradation
● Exception-oriented logic

finally pitfalls
● WILL BE EXECUTED ALWAYS!!!
○ Except for that moment, in which you will really need this...
● Might cause performance degradation
● Newer return value in finally
○ But sometimes...

Early exception handling
Exception handling
public int someMethod(String arg1, String arg2) {
final int defaultValue = 42;
int value = 0;
try {
int int1 = Integer.parseInt(arg1);
value = int1 / int2;
System.err.println(ex);
return defaultValue;
}
return value;
}

Exception handling
int value = 0;
try {
value = int1 / int2;
System.err.println(ex);
}
return value;
}
if (notNumber(arg1) || notNumber(arg2)) {
}
if (int2 == 0) {
}
int value = int1 / int2;
return value;
}

● Objects.requireNonNull(argument); (throw early)

● Use IllegalArgumentException() (throw early)

● Use UnsupportedOperationException() (throw early)

● Delegate exception handling, use throws (catch late)

● Delegate exception handling, use throws (catch late)
● Write javadoc
● Write logs

Why null is bad?
● More than 2500 NPE defects in our Jira

Why null is bad?
● null is low-level concept

Why null is bad?
● Zero information

Why null is bad?
● null instanceof Type --> false

Why null is bad?
● int value = (Integer) null; --> NPE

Why null is bad?
● Many methods uses .toString() --> NPE
○ String.valueOf(obj) - solves this issue

Why null is bad?
● Many methods uses .toString() --> NPE
○ String.valueOf(obj) - solves this issue
● Null-oriented programming is legacy
○ Use Optional<T> instead

Lean refactoring
● Don’t broke tests (Red-Green-Refactor)

Lean refactoring
● Don’t change signatures

Lean refactoring
● Don’t change parameters order
○ Especially for constructors

Lean refactoring
● Don’t change contract

Lean refactoring
● Don’t change contract
● Use @Deprecated

Abusing Java
Just because you can, it doesn’t mean you should...
● Use enum as Singleton

Abusing Java
● Use static as Singleton

Abusing Java
● Use reflection
○ Especially for internal JDK classes

Abusing Java
● Use reflection
● Use interface just because of signature

Abusing Java
● Use reflection
● Use inheritance as aggregation/delegation

Abusing Java
● Use reflection
● Use inheritance as aggregation/delegation
● ...
● If it works, it doesn’t mean it’s good.

SOLID in the nutshell
● S - Single responsibility

● O - Opened for extension, but closed for changes

● L - Liskov substitution, children should keep parents
contracts.

contracts.
● I - Interface segregation is better than God-interface.

contracts.
● I - Interface segregation is better than God-interface.
● D - Dependency inversion, build dependencies over
abstractions, not over implementations.

Q&A Session (the end)
Attention!
Thanks for your attention!

Java Tips, Tricks and Pitfalls

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