java - Is it ever justified to have an object which has itself as a field? -

is ever justified have object has field :

class thing {      thing field;      public thing() {         this.field = this;     } } 

i'm not talking class field of same type class made every instance of class has field. saw in legacy code (this field never used) i'm curious. legit use of ?

yes, though rare. used in jdk situations field this might not be.

from class implements collections.synchronizedcollection(c)

static class synchronizedcollection<e> implements collection<e>, serializable {     private static final long serialversionuid = 3053995032091335093l;      final collection<e> c;  // backing collection     final object mutex;     // object on synchronize      synchronizedcollection(collection<e> c) {         this.c = objects.requirenonnull(c);         mutex = this;     }      synchronizedcollection(collection<e> c, object mutex) {         this.c = objects.requirenonnull(c);         this.mutex = objects.requirenonnull(mutex);     } 

in case mutex might current class, if collection obtained existing synchronised collection, mutex might different. e.g. if call map.values() , map synchronizedmap mutex map not collection.

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another example throwable points cause default.

/**  * throwable caused throwable thrown, or null if  * throwable not caused throwable, or if causative  * throwable unknown.  if field equal throwable itself,  * indicates cause of throwable has not yet been  * initialized.  *  * @serial  * @since 1.4  */ private throwable cause = this;