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Find Java Bugs with FindBugs

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This post is regarding a static analysis tool that finds defects in Java programs. Static analysis tools can find real bugs and real issues in the code. We can effectively incorporate static analysis into our software development process.


FindBugs is an open source static analysis tool that analyzes Java class files, looking for programming defects. The analysis engine reports nearly 300 different bug patterns. Each bug pattern is grouped into a category (e.g., correctness, bad practice, performance and internationalization), and each report of a bug pattern is assigned a priority, high, medium or low.

Let’s start with some of the selected bug categories with the examples.


Comparing incompatable types for equality

Consider the following code,

if ((!value.equals(null)) && (!value.equals(""))) {
Map spaces =
(Map) vm.get(SpaceConstants.AVAILABLESPACEMAP);
One would expect that the condition would true, when value is not null and is not empty. However, value.equals(null) according to the contract of the equals() method, would always return false.

Consider the another similar example,

if ((bean.getNoteRate() != null) &&
!bean.getNoteRate().equals("") &&
(bean.getNoteRate() > 0)) {

We might expect that the condition would true, when noteRate is not null, not empty and is greater than 0. However, the condition would never be true.

The reason is that bean.getNoteRate().equals("") would always return false regardless of being equal value. According to the contract of equals(), objects of different classes should always compare as unequal; therefore, according to the contract defined by java.lang.Object.equals(Object), the result of this comparison will always be false at runtime.

Null pointer dereference

Consider the following code,
if ((list == null) && (list.size() == 0)) {
return null;

This will lead to a NullPointerException when the code is executed when list is null.

Suspicious reference comparison

Consider the following code,
if (bean.getPaymentAmount() != null
&& bean.getPaymentAmount() !=
currBean.getPrincipalPaid()) {
// code to execute
This code compares two reference values (Double paymentAmount) using the != operator, where the correct way to compare instances of this type is generally with the equals() method. It is possible to create distinct instances that are equal but do not compare as == since they are different objects.

Doomed test for equality to NaN

Consider the following code,
if ((newValue == Double.NaN) || (newValue < 0d)) {
// the code to execute
This code checks to see if a floating point value is equal to the special Not A Number value. However, because of the special semantics of NaN, no value is equal to Nan, including NaN. Thus, x == Double.NaN always evaluates to false. To check to see if a value contained in x is the special Not A Number value, use Double.isNaN(x) (or Float.isNaN(x) if x is floating point precision).

Also see How can you compare NaN values? .

Method whose return value should not ignore

string is immutable object. So ignoring the return value of the method would consider as bug.
String name = "Muhammad";
if (name.equals("MUHAMMAD"))


Method invokes inefficient Boolean constructor;use Boolean.valueOf(...) instead

Consider the following code,
if ((record.getAmount() != null)
&& !record.getAmount().equals(
new Boolean(bean.isCapitalizing()))) {
// code to execute

Creating new instances of java.lang.Boolean wastes memory, since Boolean objects are immutable and there are only two useful values of this type. Use the Boolean.valueOf() method (or Java 1.5 autoboxing) to create Boolean objects instead.

Inefficient use of keySet iterator instead of entrySet iterator

Consider the following code,
Iterator iter = balances.keySet().iterator();
while (iter.hasNext()) {
// code to execute
This method accesses the value of a Map entry, using a key that was retrieved from a keySet iterator. It is more efficient to use an iterator on the entrySet of the map, to avoid the Map.get(key) lookup.

Method invokes inefficient Number constructor; use static valueOf instead

Consider the following code,

Integer number1 = new Integer(123);
Integer number2 = Integer.valueOf(123);

System.out.println("number1 = " + number1);
System.out.println("number2 = " + number2);
Using new Integer(int) is guaranteed to always result in a new object whereas Integer.valueOf(int) allows caching of values to be done by the class library, or JVM. Using of cached values avoids object allocation and the code will be faster.

Also see:

Method concatenates strings using + in a loop

for (int x = 0; x < exceptions.size(); x++) {
errorMessage += getStackTrace(
exceptions.get(x) + "\n");

In each iteration, the String is converted to a StringBuffer/StringBuilder, appended to, and converted back to a String. This can lead to a cost quadratic in the number of iterations, as the growing string is recopied in each iteration.

Better performance can be obtained by using a StringBuilder explicitly.


Code that is confusing, anomalous, or written in a way that leads itself to errors. Examples include dead local stores, switch fall through, unconfirmed casts, and redundant null check of value known to be null.

instanceof will always return true

The instanceof test will always return true (unless the value being tested is null)

NodeList nodeList = root.getElementsByTagName("node");

int nodeListLength = nodeList.getLength();

for (int i = 0; i < nodeListLength; i++) {
Node node = nodeList.item(i);
if (node instanceof Node &&
node.getParentNode() == root) {
//do code


Test for floating point equality

Consider the following code,

private double value = 0d;

if (value > diff) {
// code to excute
} else if (value == diff) {
// code to excute
The above code compares two floating point values for equality. Because floating point calculations may involve rounding, calculated float and double values may not be accurate. For values that must be precise, such as monetary values, BigDecimal would be more appropriate.

See Floating-Point Operations

Also see Effective Java 2nd Ed, Item 48:Avoid float and double if exact answers are required

Integral division result cast to double or float

Consider the code,

int x = 2;
int y = 5;

// Wrong: yields result 0.0
double value1 = x / y;


This code casts the result of an integral division operation to double or float. Doing division on integers truncates the result to the integer value closest to zero. The fact that the result was cast to double suggests that this precision should have been retained. We should cast one or both of the operands to double before performing the division like
// Right: yields result 0.4
double value2 = x / (double) y;


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Using the FindBugs Ant task.

Using the FindBugs Eclipse plugin.




Published at DZone with permission of Muhammad Khojaye, author and DZone MVB.

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