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copied!<p>Immutable objects are objects that can't be changed programmatically. They're especially good for multi-threaded environments or other environments where more than one process is able to alter (mutate) the values in an object.</p> <p>Just to clarify, however, StringBuilder is actually a mutable object, not an immutable one. A regular java String is immutable (meaning that once it's been created you cannot change the underlying string without changing the object).</p> <p>For example, let's say that I have a class called ColoredString that has a String value and a String color:</p> <pre><code>public class ColoredString { private String color; private String string; public ColoredString(String color, String string) { this.color = color; this.string = string; } public String getColor() { return this.color; } public String getString() { return this.string; } public void setColor(String newColor) { this.color = newColor; } } </code></pre> <p>In this example, the ColoredString is said to be mutable because you can change (mutate) one of its key properties without creating a new ColoredString class. The reason why this may be bad is, for example, let's say you have a GUI application which has multiple threads and you are using ColoredStrings to print data to the window. If you have an instance of ColoredString which was created as</p> <pre><code>new ColoredString("Blue", "This is a blue string!"); </code></pre> <p>Then you would expect the string to always be "Blue". If another thread, however, got ahold of this instance and called</p> <pre><code>blueString.setColor("Red"); </code></pre> <p>You would suddenly, and probably unexpectedly, now have a "Red" string when you wanted a "Blue" one. Because of this, immutable objects are almost always preferred when passing instances of objects around. When you have a case where mutable objects are really necessary, then you would typically guard the objet by only passing copies out from your specific field of control.</p> <p>To recap, in Java, java.lang.String is an immutable object (it <em>cannot</em> be changed once it's created) and java.lang.StringBuilder is a mutable object because it can be changed without creating a new instance.</p>

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