Is the InputStream "int read(byte[] b)" method-parameter a reference?

Question

I am recently wondering how functions like the InputStream int read(byte[] b) method are working. I know that Java is always passing method parameters by value, and not by reference.

Knowing this, I don't understand how an empty byte-array can contain values after passing it to the read-method (assuming there were bytes to read in the InputStream). From all I know, only the value passed to the read-method will be touched internally and the only output should be the number of read bytes (the integer return value). But still the byte-array somehow fills.

How is this actually done?

The reference is passed by value, but it's still a reference to the original byte array, so it can be used to fill it. — Kayaman
– Kayaman, Commented Mar 26, 2015 at 6:55

Thomas Stets · Accepted Answer · 2015-03-26 06:55:45Z

2

The reference to the byte[] is passed as value. Pass by reference would mean that the address to the variable holding the reference to the byte[] was passed.

answered Mar 26, 2015 at 6:55

Thomas Stets

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Antimony · Accepted Answer · 2015-03-26 06:55:32Z

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Objects like arrays are passed by reference. You can think of it as the object variable being a pointer and that pointer being passed by value, which amounts to the same thing. It's only primatives like ints and floats that are really passed by value.

answered Mar 26, 2015 at 6:55

Antimony

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8 Comments

Chris Over a year ago

Are the primitive types the only exception that are actually passed by value?

Antimony Over a year ago

@Chirs yes. Though note that for immutable objects like Strings, pass by reference and value are almost equivalent.

Louis Wasserman Over a year ago

Nope. The reference is passed by value, and this is different from pass by reference and from pass by value.

Antimony Over a year ago

@Louis What's the difference? I don't think there is any difference at the abstraction level of Java.

Louis Wasserman Over a year ago

@Antimony: stackoverflow.com/questions/40480/… explains some of the details, but pass by reference supports swapping and Java does not. The distinction is important.

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