Examples of GoF Design Patterns in Java's core libraries?
 

Abstract factory (recognizeable by creational methods returning the factory itself which in turn can be used to create another abstract/interface type)

javax.xml.parsers.DocumentBuilderFactory#newInstan ce()
javax.xml.transform.TransformerFactory#newInstance ()
javax.xml.xpath.XPathFactory#newInstance()
Builder (recognizeable by creational methods returning the instance itself)
java.lang.StringBuilder#append() (unsynchronized)
java.lang.StringBuffer#append() (synchronized)
java.nio.ByteBuffer#put() (also on CharBuffer, ShortBuffer, IntBuffer, LongBuffer, FloatBuffer and DoubleBuffer)
javax.swing.GroupLayout.Group#addComponent()
All implementations of java.lang.Appendable
java.util.stream.Stream.Builder
Factory method (recognizeable by creational methods returning an implementation of an abstract/interface type)

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