CODEWITHSUNDEEP
java
user1357722
user1357722

Reputation: 7658

Why is System.in declared as nullInputStream() instead of null?

In the System class, in, out, and err are static fields. These fields are declared for example:

 public final static InputStream in = nullInputStream();

Why declare nullInputStream() instead of null?

Upvotes: 10

Views: 1061

Answers (2)

JB Nizet
JB Nizet

Reputation: 691765

The source code has the following comment:

/**
 * The following two methods exist because in, out, and err must be
 * initialized to null.  The compiler, however, cannot be permitted to
 * inline access to them, since they are later set to more sensible values
 * by initializeSystemClass().
 */

In short, since System.in is a static final variable, if it was set to null, the compiler would consider it as a constant, and would replace all the references to System.in in other classes with null (that's what inlining means). Which would obviously make everything non-functional. Some native code must be used to replace the value of this System.in final value (which normally should never change) once the system is initialized.

To resume: it's used to avoid a compiler optimization that should not be made in this very particular case, because System.in is a final field that can change, which is normally impossible.

Upvotes: 10

Bhavik Ambani
Bhavik Ambani

Reputation: 6657

You are wrong.

In Java source code it is written as 

 public final static InputStream in = null;

not 

 public final static InputStream in = nullInputStream();

You can take reference of the source code for the System class here.

Upvotes: -1

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