Fun with maps and sets

TestData1.cls

class TestData {
	public String data;
	
	public TestData() {}
	
	public TestData(String data) {
		this.data = data;
	}
}

Map<String, TestData> dataMap = new Map<String, TestData>{
	'foo' => new TestData('foo'),
	'bar' => new TestData('bar')
};
    
Set<String> keySet = dataMap.keySet();
System.debug(keySet.size()); // 2
System.debug(dataMap.size()); // 2

keySet.remove('foo');
System.debug(keySet.size()); // 1
System.debug(dataMap.size()); // 1

TestData2.cls

class TestData {
	public String data;
	
	public TestData() {}
	
	public TestData(String data) {
		this.data = data;
	}
}

Map<String, TestData> dataMap = new Map<String, TestData>{
	'foo' => new TestData('foo'),
	'bar' => new TestData('bar')
};
    
Set<String> keySet = dataMap.keySet().clone();
System.debug(keySet.size()); // 2
System.debug(dataMap.size()); // 2

keySet.remove('foo');
System.debug(keySet.size()); // 1
System.debug(dataMap.size()); // 2