OCJA(1Z0 - 808) || String Objects Creation Heap and String Constant Pool (SCP) Part - 1

Summary

This video explores key differences between creating String objects in Java using `new String()` and string literals. It explains that using `new String()` creates two objects—one in the heap and another in the String Constant Pool (SCP)—while a string literal creates only one in the SCP. The video also delves into JVM memory management, garbage collection, and how objects in the SCP are reused for efficiency. The session clarifies how the heap and SCP handle string objects differently, ensuring memory optimization and object reuse.

Takeaways

• 📜 String object creation depends on the use of the 'new' keyword, and it leads to different memory allocation behaviors.
• 🏗️ When 'new String()' is used, two objects are created: one in the heap and one in the String Constant Pool (SCP).
• 💡 In contrast, without 'new', only one object is created in the SCP, as JVM reuses the object if it already exists.
• 🗂️ The purpose of the SCP is to allow the JVM to reuse objects, improving memory efficiency.
• 🗑️ Objects created in the SCP are not eligible for garbage collection since JVM keeps an implicit reference to them.
• ⚠️ Until Java version 1.6, the SCP was part of the method area or permgen space, but from Java 1.7 onwards, it moved to the heap for better memory management.
• 🔄 In cases where string literals are already present in the SCP, JVM reuses them instead of creating new objects, enhancing efficiency.
• ⚙️ When runtime operations modify strings, new objects are always created in the heap, not the SCP.
• ❌ Objects without explicit reference in the heap are eligible for garbage collection, unlike those in the SCP.
• 💾 Using 'new' ensures a new object is created in the heap even if an identical object exists in the SCP, avoiding reuse in the heap.

Q & A

What is the difference between using 'string s = new string("druga")' and 'string s = "druga"'?

Using 'string s = new string("druga")' creates a new object in the heap memory each time, while 'string s = "druga"' utilizes the string literal pool in the string constant pool (SCP), reusing the same object for the same literal, thus saving memory.

Why is it said that using the 'new' operator is dangerous in the context of strings?

Using the 'new' operator for strings can lead to unnecessary object creation in the heap, which might not be garbage collected immediately due to implicit references maintained by the JVM, leading to increased memory usage.

What is the role of the string constant pool (SCP) in Java?

The string constant pool (SCP), also known as the string intern pool, stores literals to ensure that each string literal is unique in memory, thus saving space and improving memory utilization.

How does the JVM handle string literals?

The JVM checks if a string literal already exists in the SCP before creating a new object. If it exists, it reuses the existing object; if not, it creates a new object and adds it to the SCP for future use.

What happens to the object created with 'new string("druga")' in the SCP?

When a new object is created using 'new string("druga")', a copy is also maintained in the SCP for future references, but the original object in the heap is not eligible for garbage collection due to implicit references by the JVM.

Why are objects created with string literals not immediately eligible for garbage collection?

Objects created with string literals are not immediately eligible for garbage collection because the JVM maintains an implicit reference to them in the SCP, ensuring they are available for future use.

What is the impact of Java version on how the string constant pool is managed?

Prior to Java 7, the SCP was part of the permanent generation in the heap. From Java 7 onwards, the SCP is part of the heap itself, allowing it to expand and contract as needed for efficient memory utilization.

Can you provide an example of how many objects are created with 'string s1 = new string("druga"); string s2 = "druga";'?

In this example, three objects are created: one in the heap for 's1', one in the SCP for the literal 'druga', and another in the heap for 's2' if it's a new object not found in the SCP.

What is the significance of the string pool in memory management?

The string pool is significant in memory management as it prevents the creation of duplicate string objects for identical literals, thus conserving memory and enhancing performance.

How does the JVM decide whether to reuse an object from the SCP or create a new one?

The JVM checks if the content of the string literal already exists in the SCP. If it does, it reuses the existing object; if not, it creates a new object and adds it to the SCP for potential future reuse.

What is the difference between the heap area and the string constant pool (SCP) in terms of object creation?

The heap area always creates a new object when using the 'new' operator, whereas the SCP may reuse existing objects for identical string literals, thus optimizing memory usage.