Code Style

Apache Cassandra follows Sun’s Java coding conventions with the modifications described on this page.

General Conventions

Braces on new lines: { and } are always placed on a new line, unlike the Sun convention
Line length: Try to keep lines under 120 characters, but use good judgment — slightly exceeding 120 is better than splitting a line with no natural breaking point
Indentation: Use 4 spaces, never tabs
Trailing whitespace: Clean it up in a separate patch, or leave it alone — avoid mixing whitespace changes with functional changes

Multiline Statements

When splitting inside a method call, use one line per parameter and align:

SSTableWriter writer = new SSTableWriter(cfs.getTempSSTablePath(),
                                         columnFamilies.size(),
                                         StorageService.getPartitioner());

When splitting a ternary, use one line per clause and carry the operator:

var = bar == null
    ? doFoo()
    : doBar();

Exception Handling

Never write catch (…) {} or catch (…) { logger.error() } just to satisfy compile-time exception checking. Always propagate the exception or throw RuntimeException (or AssertionError if it "can’t happen").
Avoid propagating checked exceptions that no caller handles. Rethrow as RuntimeException or IOError.
logger.warn() is often a cop-out: decide whether the condition is an error or not.
If you genuinely know an exception indicates an expected condition, you may ignore it — but explain why in a comment.

Boilerplate

Avoid redundant @Override annotations when implementing abstract or interface methods
Do not implement equals or hashCode unless actually needed
Prefer public final fields to private fields with getters
Prefer constructor initialization to setters
Avoid redundant this references
Do not extract interfaces unless you need multiple implementations
Always include braces for nested levels of conditionals and loops; braces may be omitted for single-level only

Import Ordering

Imports must follow this order:

java
<blank line>
com.google.common
org.apache.commons
org.junit
org.slf4j
<blank line>
everything else alphabetically

Good:

import java.io.IOException;
import java.util.List;

import com.google.common.collect.ImmutableList;
import org.junit.Test;
import org.slf4j.Logger;

import org.apache.cassandra.utils.FBUtilities;

Bad:

import org.apache.cassandra.utils.FBUtilities;
import java.io.IOException;
import org.slf4j.Logger;
import com.google.common.collect.ImmutableList;

IDE Format Files

Code style files are available for both major IDEs:

IntelliJ IDEA: Run ant generate-idea-files to apply the Cassandra code style automatically
Eclipse: cassandra-style-eclipse on GitHub

License Compliance

All source files must include the Apache License header. The project enforces this using the Apache RAT (Release Audit Tool).

Use this Java header template:

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

RAT checks the following file types: .bat, .btm, .cql, .css, .g, .html, .iml, .java, .jflex, .jks, .md, .mod, .name, .pom, .py, .sh, .spec, .textile, .yml, .yaml, .xml.

If a file is missing the license header, the build fails with:

Some files have missing or incorrect license information. Check RAT report in build/rat.txt for more details!

Third-party libraries in lib/ must be Apache License compatible and documented in lib/licenses/.